[sjspry]

I can't find the related documentation/source (I'll keep looking and revise my answer as necessary), but it's probably not possible to do with the pre-made TankDrive class. And actually, I see more in common with the ArcadeDrive and your desired style than Tank.

I would say that you'll either need to find some code a previous team wrote or extend Tank/ArcadeDrive (or just starting from scratch, it'll end up being a combination of both, really). And take the ArcadeDrive code but set the two sets of motors depending on the combination of x,y values.