Yep, I agree. The two batts will work fine. The smaller one simply adds to the capacity and detracts from the load of the "main" one.

The average current will be proportional from the two batts of course. The beginning and ending (or spikes and non currents) may not quite be proportional. If we had a 20C batt that we were pulling 20C from may not be good if we adde a 10C smaller battery to since it would also pull the 10C battery at something between 10 and 20 C.