Basically Merchants can bug if you go to one at the same time, we immediately stop the trade and one of us goes and does something else. Whatever you do immediately cancel the trade if it happens, only bad things come from continuing
The point of coop is that each person controls 1 of the heroes, chaining would be bad.
Yes the archer skill is totally op, usually my friend "retreats" into the biggest mob pile and then lets lose, cutting the enemies numbers in half, he just recently upgraded to a crossbow and its just stupid :P
Other things in Coop:
What he does in town should not affect me, this somewhat works but not always. If he robs a guy and gets caught (oh gee does he love pickpocketing :P ) I shouldn't have to suffer the consequences... sure I'll rez him but punish him not me :P
Pyramids rock for Coop! Red Pyramid should also be "pickupable" if its dropped, or block both from dropping (you can get through most locked doors by tossing one pyramid through the door :P ... or even using the ranger "retreat" skill through doors)
The "frozen" effect should spell out that it causes the enemy to not only become near immune to fire but also to physical attacks...
Lightning strike causes the mage to dash into the target...