How can I figure out why the low voltage furnace fuse blows on heat but not cool?
Since you mention an igniter I'm going to assume a gas furnace. So start by turning off the gas. Then disconnect everything else other than the power in and the thermostat wiring. 

Now turn on the thermostat. If the fuse doesn't blow, reconnect one thing (like the igniter) and try again. Repeat until something blows the fuse. 



Since you have isolated the problem to the igniters, I would try just replacing one or both of them. That's probably cheap to do. If that doesn't work, then you're probably looking at replacing the control board.