You will generally find that a second comm antenna should be mounted on the opposite side of the fuselage from the first comm antenna. If that is not possible, 4ft separation is what I would recommend. The comm radios have very sharp filters so the only real risk is jamming the receive comm with the transmit comm when they are tuned with little frequency separation ( a few MHz). I have seen dual comm antennas spaced much closer than 4 feet and I suspect for those installations the user rarely, if ever, tuned the two radios to nearly the same frequency.
My experience comes from 40 years of designing antennas for aircraft at a large aircraft manufacuring company.