I use one hide, and it is just large enough to where my BP can be curled up totally over the 4" flexwatt or in the middle or totally on the cool side.

This brings up another question in the same vein of the OP's: What size hide? snug and barely big enough to fit in seems to be the consensus, but some, like me, use larger to allow thermoregulation within the single hide, or for other reasons.

Some will say my hide is too big, but my BP has a day-night routine you could set your watch by, and never has refused a meal. Others might not be so lucky I guess.