Make it impossible to have fire and cold resistance intrinsically at the same time. If you eat a corpse that gains you one, you lose the other. Things like crowning that give you both now do not give either (unless perhaps you have neither, in which case you get a random one).

Extrinsic fire and cold resistance are not affected. This makes it valuable to have those extrinsics from a ring, underused colors of dragon armor, or an artifact.

All polymorphable monsters that currently resist both fire and cold must have one of those resistances taken away; otherwise you could have both at once by polymorphing into them.