In the process of creating a homebrew system, I have run into the problem of my players wanting to be able to combine their spellcasting prowess in a show of superiority and might ( and perhaps a bit of awesomeness ). I have not, however, found solid mechanical way of adjudicating this. I have a "standard" combine actions bonus, but that does not seem to carry over the flavor of two mages summoning up their will and throwing "earth+fire=magma bomb" sorts of spells out to slug their
victims enemies with, both in off-the-cuff spontaneous casting and forewarned-is-forearmed rituals.
I have looked through a few systems, but have yet to find any with decent - or even existent - multiple caster or combined magic rules. Red Wizard of Thay in DnD 3.0/3.5 is one, Ars Magica 4e had what amounted to little more than a footnote, both oWoD and nWoD Mage was of no help, ditto on Thaumaturgy from oWoD Vampire. I have thought to double check Dresden Files RPG, but loaned out my books (not that I recall anything of the sort from there).
I find myself out of ideas to try and books to search.