The Arcane Companion doesn't specify (one of its many flaws), but I thought I recalled a ruling. I may be wrong.
Ah, John W. Curtis wrote in what does not appear to have been an official ruling, but only a "how I do it", in an Oct. 11, 2000 post to the mailing list:
And yes... btw, the racial mods are added together
(just like they are for Arcane RRs)... so if you race
has a -5 to Essence and a -5 to Channeling, you have a
-10 to resist a Sorcerer's base spell (plus your stat
bonuses of course).
Note that he *assumes* this is the way Arcane RRs are done and he uses that as his model for Hybrid RRs, which was the subject. Given that he was co-Designer and Editor on the Arcane Companion, I think it is fair to say that this was the *intent*, even though the sloppy execution of ArcCo makes it not an actual rule.