Would it be legal to have one of your alliance partners drive onto your robot, and then drive around with them on top so they could score on higher rows?
No. I believe robot stack is allowed during the End Game period and only can be done in your alliance end zone.
<G35>
A ROBOT may attach to and/or climb onto a ROBOT on its own ALLIANCE. A ROBOT
may not attach to and/or climb onto a ROBOT on an opposing ALLIANCE (doing so will
be interpreted as an attempt to damage an opposing robot, and may be penalized as
such)
Of Course 
This year the game is geared toward team work more than ever.
i would have to say legal along as if your in the home zone and dont exceed the 72" rule once out of the home zone. and i don’t see how this could be even a ounce of usefull? there owuld then be a 3 V 2 match up and i doubt it wil be more productive then two opposing robots.
This says nothing about home zone or end game… it just says it is legal to climb onto your own alliance’s robot.
You can even start stacked.
Your robot can extend 72 wide by 72 long after the start of game, but your robot can extend to the roof if it can and wants to. Even if another robot stacks on top of you, your robot and thier robot are two robots, so it wouldnt matter either way.
It would be extremely useful if: a robot that can only reach the 1st and 2nd rows from the floor drives onto a team solely built for the last 15 seconds to have a robot drive on top of it and defensive manauvers has it drive on top of it, it could reach the top 3rd. Plus it helps emphaize team work.
This is allowed…weird…but allowed…
http://forums.usfirst.org/showthread.php?t=1280&highlight=start
Not really wierd, could be strategy.
This helps score points if you have an alliance robot who is inoperable at the time due to damage or something. You at least get the 15/30 points for having them lifted at the end.
Hmm this is an interesting rule.