I see a few options here:
1) If the plumbing is dead center of the 60" and you use the two 30" vanities you could remove just enough of the sides of each vanity to make the plumbing fit. This would still leave plenty of material for support, especially if the cabinets are anchored to the wall above where the material is removed.
2) If you are putting a 60" vanity in, whether it be one or two vanities, remove the sheet rock, and re-plumb to where the two sinks will be. You would just need a tee and a few elbows. The trick here is to make sure you slope back to the main pipe. If the vanities are going to cover the wall this is definitely the way to go. You will have less plumbing running through your storage space.
You asked if it would look funny to put two 30" vanities together.
All cabinets are modular and for this reason I don't think that it will look funny. Sounds like you would have a bank of drawers on the left, two doors in the middle, and another bank of drawers on the right. The only think you may need to watch for is the swing of the doors. You would want the doors two swing apart not together. If they don't swing apart it may be as easy as removing the hinges and attaching to the other side. As long as the doors swing away from each other, and the gap between the two doors is not too large, I don't think you would be able to tell that it is actually two vanities.
Last edited by jjmill1980; 02-02-2010 at 10:17 PM.