For two bikes, I would probably say one long and one short. One short one to lock one bike down to something secure and the longer one to lock up to the other bike. Or, if needed, they can be used separately. If you can get the top tubes really close together (panniers and such might get in the way of this), you could probably get away with two short or one long, but I'd say go with an option where you can lock separately as well.

EDIT: I forgot to mention, some cheaper u-locks use a simple stamped steel sheet retainer. The nicer (but not necessarily more expensive) ones use actual solid pins on each side. Imagine if the striker on a door knob was a stamped steel sheet as opposed to a solid chunk of metal. Normally, you have to open the locks to check this out, but I've had good luck with brands like On Guard and Bulldog. Kryptonite is usually great, but generally more expensive.