Actually was thinking why include a Enspell strike? Why not have a simple system of magic > magic = burst. Or call it something like "euclidation". Basically, either a RDM can cast off themselves to create a desired burst or off another casters magic.
Rather then mix and matching.. I think a linear progression only...for ex, thunder III > thunder II = what's the thunder combo impaction? I would say III > II because opposite might take too long to successfully link.
Ifwe're talking about casting off another caster, then simply casting a like- spell right on top of their cast could "burst"