I recently imagined a creature with many diverse traits, of which I will only be focusing on one of. Could a creature feasibly have two knees on each leg? Could it be unhindered, or maybe even benefit? It maybe would also be digitigrade, looking something like this either way:
There are animals on Earth with multi-jointed legs. Crabs come to mind, with upwards of 6 joints. Those joints, however, are restricted to move only in certain planes.
A mammal-like creature with two knees would hopefully also have two elbows. This creature would likely be able to articulate itself well in constrained spaces, possibly during climbing. It may have an easier time changing its leg shape to get purchase on rocks or in dense foliage, since it has multiple options of where to bend and therefore multiple options of how far away to find a foothold. It may similarly be able to swing or maneuver a bit easier, since it also has options for the radius of its swing.
We might imagine such a creature being able to lock its bottom knee joint, essentially creating a longer foot and appearing, as you said, to be digitigrade. That would be beneficial if it were living in bogs, for instance, where an upper knee is helpful for being able to move even if the lower leg is fully engulfed in water or mud. On more solid ground, the creature might use the lower knee joint instead to take advantage of two sets of muscles to move the leg, perhaps being able to exert greater strength.
I personally envision this creature as being able to switch between bipedal and quadrupedal motion modes, and to be able to choose between "dog-like" and "crab-like" quadrupedal motion depending on how it intends to articulate its joints. It might be an excellent runner in dog-mode, but a fantastic climber in crab-mode, and an efficient long-distance roamer in bipedal mode.
There's nothing improbable about it. Actually, arguably there are already creatures on our planet that have this sort of limb: birds.
Their metatarsus is technically a fused foot bone, but at this point, it is distinct enough from a typical digitigrade stance, so I think it can count as it's own thing. If the bones would fuse a bit differently, they would form a metatarsus and a true foot platform, or perhaps the toe's first segments can fuse to take up that function instead.
From a biomechanics point of view, maybe consider the different planes of motion that the joints move in.
In us humans, our hip joint has more range of motion in all 3 planes than most other joints, with the notable exception being the shoulder.
Our hips have huge amounts of rotation in the transverse plane, huge amounts of forwards/ backwards motion in the sagital plane, and slightly less but still impressively large lateral ability in the frontal plane.
(This is why physios and trainers always advise to use the hips where possible for most functional lifting instead of the lower back - because at each of the 5 lumbar joints there is only something tiny like 3-5 degrees of movement in any of the 3 planes, totalling out at about 15-19 degrees of movement over those 5 joints - whereas the hip gets close to or goes over something like 150-180 degrees in all 3 planes.)
Following on down from the hip we have 1 large bone, the largest and longest bone in the body, the femur/ thigh bone, which lower down joins with 2 bones, the tibia/ shin bone (2nd longest) and fibula, at the knee.
Now, the knee is very limited in both lateral and transverse movements, especially when in a straightened position.
When bent it gains a little ability in both of those, (and all movements in any of the joints usually involve all 3 planes to at least some degree), but primarily the human knee, a hinge joint, is a straight up sagital plane, forwards and backwards work horse, designed by evolution not only to accelerate us but more importantly to decelerate us and to stabilise us as we change direction, to mitigate our body weight vs other things like gravity, ground reaction force, inertia, friction, etc, in, as I said, primarily the sagital plane.
Then, continuing down we have 2 bones, which end at the ankle, another hinge joint intially as the tib and fib meet the talus, where we then splay out into many bones and many diferrent kinds of joints.
The point of all of this maybe excessive anatomical exposition is to highlight what constant gravity and repetitive functional movements over millions of years have done to our body shape.
Our legs go 1 bone > 2 bones > many bones (as do our arms) in order to fulfill certain functions like locomotion, weight bearing, etc.
That said, you should perhaps be mindful to look at how many extra bones you would need to introduce downstream from the 2nd knee.
Or would you break the existing femur in two and add another joint there?
Or would the lower knee be like the hip joint, able to move in virtually all directions?
Or would the 2nd knee joint have some other specific role like sideways movement, or tree climbing, or to turn the lower leg into an aquatic propeller of some sort?
I must admit I don't initially know which way I would go in designing such a joint nor where on the limb to place it, but as hinted at earlier, my thought process would be to 1st understand the evolutionary requirements of that creature as a whole, what did it need to do and in what type of environment(s), and then work from there.
Please forgive the biomech rambling - it is what I have been doing for the last 30 odd years and I tend to natter on a bit - apologies!