Different Van's Answer
I called Van's just the other day about this and spoke with Ken - and got a 20 minute lecture on how I was putting way too much stuff in the airplane if I had already filled up the 4 holes they give us. He actually said, "I've never seen an RV with two radios in it." ... and that, "That's as crazy as putting in something like a VOR."
In the end he relented to my unashamed deviation from "Van's intentions" for the plane and told me an extra hole would be ok if it was at least 1.5" from the others and best if it stayed near a doubler.
Probably best place is directly above the one called for in the spar that lies so close to the out board doubler that the plans have you shave off a flat on the snap bushing.