I've always averaged the bonuses. . .I've been wrong in the past though, it appears I am wrong in this instance, per the RAW.
As a point of reference. . .in my games when you resist Channeling it's at In bonus (or x3 IN bonus) and when you resist Essence it's at Em bonus, but when you resist a Sorcerer's Essence/Channeling hybrid lists, you resist them at the average (In/Em)/2 i.e. (inx3)+(emx3)/2
I think that makes sense, and establishes a precedent, in the sense that if you're averaging the racial stat bonus, you should be averaging the RR bonus, which gives you bonuses all around the same range, rather than have the bonuses for hybrids be radically higher or lower than for pure.
Rdan is correct above, the reference is in RMFRP Character Law page 16.
Hybrid caster casting a spell from a pure list, it's resisted as the pure single realm as normal.
For hybrids casting spells off hybrid base lists . .rather than (Emx3)+(Inx3)/2, it says to use Em+In, and that if either realm of the hybrid spell corresponds to the target's realm they get the +15 own realm bonus, and you use the BAR column that's most to the target's advantage. . .
RMSS Spell law uses the text lifted from RM2 SL, so I don't think it applies, especially since the RMSS core book text seems to match the RMFRP CL text.
So it seems Peter's comment above makes sense, though it makes Hybrid lists very hard to use on many targets, especially them Dwarves, and makes the bonuses vs hybrid lists scale somewhat differently from pure RR bonuses.
I couldn't find any actual book reference explicit in the racial RR bonuses vs Hybrid spells though, it only discusses the stat bonuses. . .if anyone finds the reference, or the errata, a point there would be much appreciated.