If you decide to mount the antennas externally, please be aware that Van's advises against drilling any holes in the RV-10 fiberglass canopy that are not called for in their plans, as the cabin top is a structural element of the aircraft. If you follow this advice, it precludes holes for antennas.
I was able to fit both GPS antennas, plus a com antenna and an ELT antenna on the aluminum tail cone top of my RV-10, while maintaining recommended ground planes and distances between antennas.
I realize there are a number of RV-10s with antennas mounted through the fiberglass top, and as with all things in Experimental aircraft it is up to you as the builder to decide where you want to mount them.
RV-10 Finishing Kit & FWF in-progress
2019 VAF Dues Paid