Can you use two elements to make a spell... strictly, no... but then again yes. If you're trying to toss molten rocks around you can, reasonably, say you've speed up the atoms in the rocks, caused them to shift around, creating a "fire" affect will never venturing outside the earth element (much like using earth to make lighting), letting you use your earth power and control for a fire spell. Or, if you wanted, you can take a turn, make a maneuver on one action, cause the earth to heat up boiling rocks making the temp aspect of Boiling earth (or something) then next turn make an earth attack with the free tag you can do it that way.