Why not use a rechargeable C cell? They're readily available in both NiCd and NiMH versions.

Otherwise, from an electronics standpoint, two cells operating in parallel should work fine. The glow plug only cares about voltage, not the number of cell providing that voltage. Just be careful of drawing it down too far. Although not as risky as Li-Poly batteries, you can get cell reversal in NiCD and NiMH that will destroy the batteries. Just don't go below 1 volt.

Brad