TL;DR: Nothing save the ability to accurately make the jumps needed. However, beware Consequences.
Basically, just because you can do something, does not mean that you should do it.
The Cold War
The two groups are in a cold war. Likely they are in said cold war because both sides know that a hot war is a very real possibility and one that both sides would like to avoid if possible. It would be bad for their health and bad for their people -- given that it is likely that low-value planets are likely to just get glassed than invaded conventionally.
Both sides know that the other could plausibly hyperspace jump into their territory, just as both sides know that such an action would escalate the cold war into something much worse. While each side may not know everything about the other, they likely know enough through spies, defectors, and neutral parties to have an idea of the other's capabilities. Enough to know not to try it.
This is precisely why such a demilitarized zone exists -- to prevent things from escalating too easily. If a computer glitch accidentally jumps a ship into this DMZ, then there will be tensions, but they can be talked down. Any trade between the two empires can swap in there, meaning that each group can keep their secrets while accessing luxury goods the other does not have.
Plus, there are rules to war regardless if it is hot or cold. Perhaps not written ones in this case, but I can see that there are things that both sides have mutually agreed to. A code of ethics one might say based on their desire to conquer as opposed to destroy. This unspoken code may also be helping keep the war cold, and it is very likely that a surprise hyperspace attack will violate this unwritten code.
The Hyperspace Conundrum
“Space [...] is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.” -- Douglas Adams
Those routes on the Hyperspace Map are the ones that were either found or already calculated out and are known to be the safest. They will be the routes people travel regularly because they offer the least chance of a mishap -- said mishaps possibly involving eldritch space horrors from another dimension breaking into ours and exploding heads.
These routes have either been found or calculated. They're standard so I can see them being put into the computers of basic spaceships the same way a GPS has the basic maps of the world in them. Every so often, they'll need to be updated -- but so does any map.
As these routes are known, they will be the known choke points to be defended should anybody get ideas. Anybody with a complete enough hyperspace map of the galaxy could figure out the path to drop in on the other empire should they want to, but such a thing works both ways
Consider something a bit closer to home. Alpha Centauri, our closest stellar neighbour, is 4.3 light years away. Alternatively, 40.68 trillion kilometers away to put it in units a bit more comparable to what we use on Earth.
To calculate an FTL Jump to this star system, variables to include would be:
- The distance of the trip, to a likely higher precision than 4.367 light years
- The movement of the two solar systems in the galaxy
- The contents of the two solar systems -- note that Alpha Centauri is a triple star system
- The effects of Hyperspace between the two stars
And this is just to the next star. While, yes, the sci-fi technologies likely can do this math like we punch numbers into a calculator, it all scales up in difficulty the more star systems that FTL route can potentially cross through. It take a long time to not only calculate a new route, but thoroughly test it to make sure it will be a generally safe one.
Even for such a large empire, it's not worth it to calculate a whole new route, just to drop in by surprise a couple million miles from the established exit point of a hyperspace route just for a surprise attack you'll only make once. It's probably far easier to make a bomb that can travel through hyperspace and send that to greet the defence at the exit point