My bass instructor and I are working on this right now. He told me to just practice scales whilst double thumbing. Doing two things at once is awesome.

I'm just starting to learn proper slap and double thumb, as well as scales, so it's a lot to do, but it's working.
ex. G Ionian
G|---------------4----5----7-----9----11------12-
D|-----5----7------------------------------------
A|------------22---33---55---77----99----1010----
E|--33---55--------------------------------------
-----DU P
D= down, U= Up, P = pluck