One factor that seems to me to play a huge role here, but I don't see much mention of it, are the mechanics of a socket joint vs. a hinge joint. Humans benefit from a single hinge joint attached to a socket joint in both our arms and legs, because if our hand or foot gets "stuck" rotation around the socket joint protects the weak torsional strength (e.g. rotational strength) of a hinge joint. Hinge joints can break easily when twisted. An extremely long limb with two hinge joints would be very susceptible to torsional breaks.
Therefore, your goblins would be very concerned about twists to the joint that might cause it to break; and the longer a limb, the more likely this is. So, like the praying mantis (as mentioned in one answer), these creatures would likely hold their "hands" very close with one joint nearly always "closed" and pressed against their body - to reduce the odds of breaking one or the other hinge joint. They would likely appear to have much shorter arms than they actually do.
The addition of an opposable thumb does not present exceptional advantage for anything wider that the width of a hand (e.g. two triggers on a gun??). That, however, can create some interesting side effects that I will mention in a moment.
First, consider that nearly all weapons are developed from tools used to gather food or kill prey. As these goblins are not predators, gathering tools would become their weapons of choice due to the tools' familiarity, abundance and widespread use by the race.
Thus, to develop a weapon or weapons, it is probably best to start there. First, with such lengthy arms, a quick "stabbing" gesture is likely to be developed - they could gather their food more rapidly using such a skill and also surprise enemies more easily. I'm sure they would want to extend their limb for the shortest possible time in order to minimize the risk of it twisting and breaking.
I would assume a second thumb might make it possible to wield two daggers in one hand, creating something like "scissors" - which would be incredibly effective in both hunting and fighting. It might be possible to have nearly long enough daggers such that the goblin appears to wielding 4 small swords, two in each hand. Perhaps adding a hinge to each set of daggers would be another interesting twist on the concept, so that they essentially are scissors. Although, we humans add the hinge because we cannot control both blades with two opposable thumbs, so the goblins may not need the hinge on the two blades and it could, in fact, reduce their options (like tying chopsticks together...).
Aside from the stabbing, an extra hinge could give more momentum to a weapon that swings on a short rope or chain, or is attached to a short rod or resembles a bat or club. A long cable is more likely to get tangled and risk torsional damage (e.g. it "catches" on the enemy or an object as the goblin is swinging it and then the goblin must release it or risk getting injured). Similarly a long club or bat would be unwieldy for their already long arms.
However, it is the length of the arm that gives these weapons an advantage over shorter limbed, single hinged creatures since for these goblins, both hinges could extend simultaneously making the head of the weapon move very rapidly, with great force and momentum.
The danger here is in making such a weapon too heavy - it's momentum could build so rapidly that breaks the goblin's own grip, or worse, their arm. But momentum is mass times the velocity, so the faster it moves, the less heavy it needs to be for equal effectiveness compared to a similar weapon wielded by shorter limbed creatures. And the energy it accumulates (mass times velocity squared) is an advantage by a wide margin (the square of the speed vs. linear increase based on mass). Also, force is related to mass times acceleration - so it will strike with surprising force.
So a small bat or club would be much more deadly in the hands of these goblins than most humans would suspect. If a human encountered such a creature with a small club, the same weapon for a human would be ineffective in combat, while being very effective for the goblin. Anything with protruding blades or spikes could become lodged in the enemy, again risking a break in the arm or release of the weapon. I would expect something more like a spoon (for gathering) and a bat (for crushing foods or dislodging them).
They might wield tools or weapons that lodge or have long ropes or chains when one hinge is closed, but it would be extremely risky in situations where both hinge joints are "open" or extended. Their ability to learn and use such weapons would be based on the less frequently need to construct things or creative scavenging (twisting knobs, carving, tightening, loosening, etc.). With their passive role, these types of tools would unlikely turn into weapons, but it would be possible, and it would probably resemble the way a human would wield them.
Wielding a two-handed weapon would require the hinged joints be moved in parallel in order to reinforce the joints, otherwise the additional speed of wielding the objects would put the goblin at risk of breaking their limbs. However, moving in parallel is common with swords and clubs. It would allow longer and larger weapons to be wielded. (It might also be interesting to consider how they might develop bones that "interlock" and also allow them to "cross their arms" while still bending them...)
Also, with the second thumb on each hand, the addition of a second, hinged blade or even a small sword or adjustable shield would create a weapon that has incredible potential. A secondary small blade would be very useful in close quarters combat where the opponent blocks the fall of the larger blade with their weapon, leaving them defenseless against an attack on their hands or arms from the smaller weapon. A highly skilled goblin may also have the ability to hold 1 large blade and 2 small blades, making defense against the weapon incredibly difficult for those of us only able to wield a single blade with two hands. Or even the goblin may wield to "medium" swords, but with both hands still gripped together.
For gathering, since these should first be thought of as tools, I could imagine a larger blade for stabbing a food, while the smaller blades serve to severe the large piece from being attached to something. Perhaps as a way of severing a limb from a dead (or dying) animal, since they are not hunters. Basically to speed the gathering and retrieval of food the process is: stab, severe and run. Two medium blades could be used for rapidly cutting foliage to reach mushrooms, etc.
The adaptation of tools to weapons I think is critical in developing both believable and intriguing weapons, in your case. At first, it was hard for me to see a clear advantage of the two hinges (torsional breaks are too easy, even in humans), but it might be possible with the thoughtful construction of the goblin's habits and behaviors, and then the development of their tools leading to the development of interesting weapons. Protecting hinge joints is something that we all innately have as humans, it would be more so with a double hinge-jointed goblin. To me, this is key in keeping it "real" while also exploring intriguing options.
If nothing else, I hope you enjoyed reading another opinion. :)
