If you wanted to change the battery type (alks will be 6Volts per 4 in series connection, NiMH will be 4.8Volts) you need to know the input range of the driver, if it will take the increase in volts then OK but if 4.8 is the max then putting 6 through it could distroy it.
6 volts is the
initial voltage of alkalines
4.8 volts is the
median voltage of NiMH
A freshly charged NiMH is about 1.4V, or 5.6V for 4 in series. Also, NiMH have lower internal resistance than alkalines, so that voltage number will not sag as much when connected to a load as alkalines, either. Under heavy loads, NiMH will actually sustain a higher voltage than alkalines.
I'm not trying to be nit-picky, but the NiMH voltage thing is a pretty common misconception. I've heard of people frying circuits because they connect 5 NiMH cells in series, thinking that 5 x 1.2V cells will work the same as 4 x 1.5V cells, when in fact those numbers are not really describing the same thing.
Original poster:
Connecting both your lights in parallel to a single battery pack (rather than carrying 8 "C" cells) should work fine. In fact, if you wanted to get the same runtime as you currently do but spend less money on alkalines, you could install a four D-cell pack, and run both lights on that.
The only time when this might be a problem is if by using a smaller battery pack you load it with too much current. In a high drain application, cutting battery pack capacity by half can reduce runtime by more than half, as there will be more energy lost due to resistance in the batteries. Your application is fairly low drain though (25 hours runtime), so that will not be a problem at all.
