Fame at last.
Was planning of doing a "dual energy type" attack for another conversion over on the Creature Catalog conversion and was thinking of using something like the following.
[Name] damage is both [TypeA] damage and [TypeB] damage. A target with immunity or resistance to both damage types has immunity or resistance to [Name] damage. A target that is immune to one damage type but has no immunity to the other damage type (i.e. one that's immune to TypeB but takes normal damage or is resistant to TypeA) has resistance to [Name] damage. A target that has resistance to one damage type but has no protection from the other damage type takes normal damage from [Name].
That seems a bit wordy, but it's difficult making it shorter without it becoming a little unclear.
Alternatively, it could simply do half and half damage (so effectively a 6d6 "iceflame" attack is effectively 3d6 cold and 3d6 fire) in which case the target's resistances & immunities will interact with the damage according to the standard rules.
That'd have a slightly different result though.
The "treat it as both" approach above works out to.
Immune to both => 0% damage
Immune to one, Resistant to other => 50% damage
Immune to one, full from other => 50% damage
Resistant to both => 50% damage
Resistant to one, full from other => 100% damage
Full from both => 100% damage
Splitting the damage into two damage rolls is:
Immune to both => 0% damage
Immune to one, Resistant to other => 25% damage
Immune to one, full from other => 50% damage
Resistant to both => 50% damage
Resistant to one, full from other => 75% damage
Full from both => 100% damage
Both work fine but the second requires a bit more maths.