The Tavern

Is it worth wearing two rings of fire at the same time? Do the bonuses stack?

Yes, spellpower boosts stack up to 3. Unless you're an Octopode, this cap only comes up if you're wielding a staff of fire, wearing two rings of fire, and are casting Ring of Flames.

Do try to mind your cold resistance if you're in an area where frost giants/ice dragons spawn, but if you aren't then rC- imposes the same damage ramp-up as rC-- (though you really don't want either).

Or if you're wearing a robe of the Archmagi.

If you have Fire ball or better at 9 spellpower, having rc-- is manageable to an extent. I'd be more concerned about managing the noise in that case.

Two Frost Giants, for example, would be a cause for worry.