Some words on DK2 Coop: This game is not suitable for cooperative play, but with these maps it should still be possible. You should be careful not to cut off your partner's path, though, and you will have to deal with crashes.
Crashes? Yes, networking is not that stable with dk2, but the maps are all tested and can be played without crashes - if you are somewhat lucky. However, under special circumstances the game might still crash; random, probably bad sync, bad timings.
Our "counter-measures" on Windows 10:
- apply "GIM DKII General Improvement Mod v1.05" to downgrade V1.7 to 1.5, which is said to be more stable. Google for download.
- Stop all other programs and unnecessary processes. We even reboot Windows before playing.
- disable thread-optimizing in Nvidia control panel for dk2.exe
- use this batchfile for starting dk2: start "DK2Coop" /belownormal "C:\GOG Games\Dungeon Keeper 2\DKII.EXE" "-32bitdisplay -32bittextures -nointmouse -disablegamma -32bitzbuffer -32biteverything"
- avoid possession, if possible (we sometimes watched sync-fails with this spell while networking)
Even with this steps we had random crashes, but much less than without.
ofc you may try these steps if you encounter crashes in skirmish or competitive mp.