With the cabinets properly screwed to each other, from both directions, you won't have an issue. 500 lbs evenly spread across two cabinets will be fine, especially since you attach to the wall as well through the hanging rail, even on base cabinets. If you still have concerns, place a small filler cut horizontally and used as a ledger board across the wall at the rear supporting the cabinets on that in addition to the feet. You wouldn't need more than maybe 1", and color matched to the cabinets, it will not really be visible.

Your wall and floor need to be absolutely FLAT for this look to work well. It's all about the prep. Or you will end up with one foot or the other not making contact, and gaps at the wall. It's all got to be level and plumb, starting with the floor and wall.

Thanks so much for your reply lwo. I went to Lowe's to buy the cabinets and just couldn't do it. I realized that what you said was absolutely the way to hang the cabinet so I wasn't worried about that. I just thought the vanity reminded me of an elephant perched on a beach ball.....a giant box on little stick legs, bad proportions.

It turns out they had a vanity with exactly the correct configuration but the top drawer of the drawer stack was non-functional and the drawer stack and the full height sink base did not have a separator. I really don't like when the drawers and sink base interiors blend together. I know I am being too picky but I can't help it!