Spell resolution is currently single roll (for the all but the elemental attack spells). The caster only makes 1 roll.
Or were you talking about combining the caster's roll and the target's roll? That would be slightly problematical.
The only way that I could see something like this working, is if it were something like the following (this is off the top of my head -- coming up with it as I write it):
Defender RR = 75 + Racial RR Mod + Stat Bonus + Level; caster has to make casting roll (caster has skill in list - normal skill progression, plus stats, list known to number of ranks in list) and meet or beat targets RR number...
Thus a 5th level Dwarf with an Empathy stat of 90 would have an Essence RR of 120 (75 base + 40 racial + 0 stat [+10 from the 90 & -10 from racial mod] + 5 level).
And if a caster knew the Dispelling Ways list to 8th level, he would have 8 ranks (+40, and if his Empath Stat and Re stat bonuses were both +10, and he was 8th level, that would give him a bonus of 58 (40 ranks + 10 stats + 8 level). The caster would have to roll 62 or higher (adjusted by normal casting modifiers) when casting a spell from the list in order for it to effect the target.
Tywyll - you wanna play test that for me, and let me know how it works? If it works okay, I'll add it as an Express Additions Option.
Benchmarks -- kinda hard to do when each race and creature uses that same 1-100 scale from the frog to the dragon. The numbers are relative only for a single race (or related group of races).
Next EA will contain a LOT of Shadow World races statted up for RMX -- about 44 of them. Plus a few other things. The issue after will begin a series of Conversions for using RMX with Cyradon.