Since I've played with this a bit further, I just thought I'd share my findings. I was interested to know whether in practice I could live with the failings of this circuit:
Where RB is the lower value pot, shunted by RA, and RC is the higher value pot. Taking 'pB' and 'pC' to be the variable proportion of RB and RC (ie between 0 and 1), I think the voltage divider has the two 'legs':
R1= RA||pB.RB+pC.RC and
R2= (1-pC)RC
Sticking the resulting formula in a 2-dimensional spreadsheet table (with some representative values for the various 'R's) shows what's wrong with the circuit:
The fine control works well when output is near full voltage, but as the voltage approaches 0v, the fine control ceases to do anything at all. Obvious really, when you look at the circuit. I've soldered up the pots to check my calculations, and the Real World agrees.
If I actually had a dual-gang pot to hand, I'd try George's idea, but I don't.
Leslie Green is also dead-set against using pots as variable resistors, saying they should be wired as pots 'whenever possible', but I'm stuck for how you'd wire two in series without making one of them a variable resistor.
Regards
John