What do you mean what was it like? You could still do it now without the rerelease. The thing that sucked about OG SS2 coop is that the audiologs don't play, and they're some of the best parts of the game.

https://store.steampowered.com/app/866570/System_Shock_2_25th_Anniversary_Remaster/

With the rerelease, you just have proper online.

Can't speak for the remaster, but I did play through SS2 co-op with a friend over Hamachi back in the day.

The game was clearly not designed or balanced around more than one player. You both get the same cybermodules whenever either one of you pick up cybermodules, but that is the only thing in the entire game where both players get the item. Everything else in-game will have to be divided up between the two of you. Ammo, healing items, armor, weapons, everything. So if you both wanted to rock assault rifles and grenade launchers, have fun getting enough ammo for the both of you.

>>12642282
>run out of ammo
>keep doing banzai charges with the wrench, constantly dying and respawning

>>12642498
I should mention the other half of the game not being balanced around more than one player. Even on the hardest difficulty, it makes the game much easier since you can throw out twice the firepower and down enemies much quicker, as well as attack enemies from behind while they are busy dealing with the other player. Even ninjas are less of a pain to deal with since you can catch them in a crossfire.
I guess maybe you can both go PSI an have a more challenging game, but Army+Navy will have no challenge pushing through.

>>12642282
A fucking hilarious mess, but I only did it with like 4 other retards who had no idea how to play the game for the lols. Back when we used to grief gmod servers or I would host sven coop servers for Facepunch and we would play sv_horsecock7 with a million different playermodels