At that level its a bit more of a personal issue but I think it would be wrong to have an agreement in place, sure you can ask and its likely that you would not poach each other's workers because it would damage your own relationship, however to keep this going as your company scales would definitely be wrong. You don't have to poach anyone's employees, but to have formal agreements in place not to do so is clearly collusion. This would also include summarily rejecting applications because they currently work for the other guy's startup.