Can I have two staves, one with 3/4 and one with 4/4?
Yes (next question… oh, how?)
Enter one (or each) as independent time signatures by terminating the shift-M popover with alt-Enter instead of simple Enter…
( Inputting time signatures with the popover • Dorico Pro Help • Reader • Steinberg)
Perfect. Thank you.
If you want synchronized staves perhaps a search on multiple time signatures may be interesting. One is Multiple time signature, another one is Polymeter in Bach. Probably the best is Masking meters and creating polymeters with metric modulations in Dorico - Scoring Notes
A simple example:
