We did that by hanging a single next to a double to get the length we wanted.

Thanks! How did that work with the sink plumbing? Do you remember what sizes you purchased?

Can you use a tall ~20" cabinet with the same door style on either side?

You can put floating shelves between two single vanities

or

one floating shelf and use it as a make-up space with a little chair/stool/ottoman underneath 

Or you can get a small cabinet for in-between and use... like a 20" piece of glass or something as a countertop

We got 3 30" & hung them on the wall. You need to alter the top drawer to make room for the plumbing.

I did that with three 24’s hung side by side with custom counters and under mount sinks. Two of the drawers were lost completely so only four drawers total.

Do 2 36" ones