How do fortune dice affect opposed checks? "what is less than half if I have 2 fortune dice in a characterstic such as strength?"

jh

They don't, which is one reason why stat fortune dice are very overrated, having high stats is worth it.

Houserule, but I have them become misfortune die on the role against that stat.

They don't, and probably shouldn't. They are powerful enough on the offensive side of things (especially for their cost).