First off Iowa Dave's list is good. Hello Dave.
I might add that while you can choose to reach 20 or 30 amps, combined, you can also choose not to combine, something impossible to do with a bigger single unit.
You also can reduce noise when leaving one off.
The individual units may be easier to handle than a big unit. A benefit, but do secure them!
Dave said one unit has lasted since 1984. Separate units provide you a plan B, and may save wear on one.
Last of all, in an emergency, you can rent the 2nd one to a neighbor for $500/day. Nice.