After reading this thread I am intrigued by Zubrei's suggestion to have joint-team turns.
My group has played quite a few 2v2 games with team mates sitting across table. We initially discussed joint-team play but elected not to go down this route. We wanted to avoid changing the official rules.
We did change the board though:
http://forum.arcanewonders.com/index.php?topic=14232.0).
Regarding players sitting next to one another or collaborating in secret, while the two mages are acting as a single player, they are two mages and should act accordingly. We permit cross-table talk. The two mages can talk or yell in the arena. Allowing the two mages to secretly collaborate during planning is not permitted; they can talk to each other but they must do so across the table in the presence of their opponents. Again, these mages are in combat in an arena, not sitting next to each other in the stands operating robots.
The joint-life total was also discussed early in our team-play but was quickly discarded due to the rule challenges it poses (Regen, Finite Life, Blood-thirsty, condition markers, mage specific traits, and many more). We have not experienced any problems with individual mages' life. Each mage needs to look after one another - if one mage dies early then the other mage is likely doomed.
I will revisit the joint-team play idea with my group. This seems like a very doable idea and solves one pesky challenge we've had with "passing" because you have the fewest active creatures (do you have the fewest literally or fewer than your opponent).