I've been thinking of something like shared/private projects/contexts etc ,where when defining each project -or maybe when registering (I'm using hosted Tracks) ,one can set up with whom he'd like to be able to share.

So the main idea is -when creating a project one can decide it's shared or private ,and if it's shared then both users can view/change/complete actions . This of course means that a username should be added to every action so that it can be easily seen that someone else edited a project ,etc.
Perhaps more permissions should be considered ,such as having a "lead user" for each project so that only she can add items -so that there is sort of "project owner" and the other user/s can complete actions but not add them (doesnt sound too nice somehow :))

I've been using Tracks for couple of days now and am enjoying it very much :), (Tried Next Action and GTD Tidly wiki before that ) thanks for a great app .