The answer to this one seems simple to me: Do you want to cause disadvantage to your ally-enemies ranged attacks? Then you do. Do you want to maintain your temporary truce while you both shoot the mutual enemy? Then don't go bumping shoulders, or whatever.
IF we want to really stretch this hypothetical to you are hidden, invisible, and bumping shoulders with another archer who's taking shots at the same target as you, and you want to give them disadvantage but still remain hidden from them, well, I mean, I think we've stretched to a scenario that probably won't ever exist, but I suppose if it does... it's up to your DM. Just like it always was for all our other scenarios. I'd probably let it happen, myself, because ultimately, it's so rare, who cares if it's a little generous? (It would never happen in my game with the invisible-hidden-ally-foe-hindering-a-PC's-ranged-attack. It just wouldn't happen.