For 20m I am planning a quarter wavelength vertical radiator with 4
eighth wavelength radials, mounted 12 feet high.

1/8 wl radials, with an elevated ground plane won't work
worth a hoot. They won't be resonant.

If I add another 4 radials cut an eight wavelength for 30m, will it be
easier to operate on 30?

Not really, unless you have a way to tune the radiator.
All elevated radials should be 1/4 wave long, unless a
case of using longer, such as 3/4 wave. 1/8 radials are
ok ground mount, but not elevated. Elevated radials must
be tuned and resonant to really work.
MK