The music in question is scored for two pianos. I find options for time signatures for each of the four staves and a single time signature for all four staves, but not for one time signature, each of which serves the two staves of each piano. Unless, of course, I’ve missed it. Thanks, all!
Do you mean a large time signature displayed for each piano? Or an independent time signature for each piano?
Setup menu / Layout Options / Time Signatures / Show once per bracket.
What I need is ‘once per brace’ and not ‘once per bracket’. I did try this, but it does not work for braces. 
…yet Rob’s screenshot seems to show exactly what you want.
For some reason, Dorico was not recognizing the braces in each of the two pianos (as opposed to brackets) as being things for which it could apply a large time signature encompassing both staves for each piano. Dorico in this situation is looking for brackets.
However, I wonder if that is really the case. I tricked Dorico into ‘seeing’ my piano braces by turning them into brackets by putting each piano into its own group, thereby enabling me to change the piano braces into brackets and then back to braces again.
Once I did that, the large time signatures as in Rob’s screenshot were no longer an issue.
thanks, all!
I was just about to suggest putting them into groups, which is in fact the best way to force large time signatures (among other things).
OK, it wont work if you bracket the two pianos together. But a few published piano duets I just looked at don’t have a bracket. You can get rid of it with Engraving Options / Brackets and Braces / Ensemble type / No brackets.
FWIW I made the example by taking the Solo Piano template and duplicating the Piano player.