The issue stabbey, is that the choices you make have repercussions, the drops you get will influence your gameplay, and so on... so even if two people continue on their side, when one of them join his mates game, it isnt really his game anymore depending on the amount of time played etc etc etc
and the prob with my friend was but an example, if he doesnt want to play D:OS for a while, fine by me, but continuing the game alone would kind shred coop from its meaning...
