What is trouble?
Putting anything within 1/4-to-1/2 wavelength of the antenna is asking for trouble. So, the problem is more than 3x worse at 121.5 than at 406.
There where several comments about 406 antennas being short. The New 406 ELTS still have 121.5 Mhz (for local DF). Yes the 406 antenna is short, but Artex for example has combined both 406/121.5Mhz in one antenna, which is still the same length as current 121.5Mhz antennas. Other 406 Mhz units have two separate antennas for the respective 406/121.5 freqs.
Having two antennas real close is a problem or possible trouble, but define trouble. You will polarize the signal somewhat, but it will still transmit. It's no worse than burying ELT antennas flat Horz or inside and next to metal structure. The "radiation pattern" will be very distorted. Like the empennage intersection fairing area. Yes fiberglass is RF transparent but that vert stab at 12 O'clock, horz stab at 3 & 9 O'clock and the fuselage at 5 to 7 O'clock is like kryptonite to Superman. So you hope the vert polarized antenna laying horizontal will shoot out a signal between little slots you left. I'm a libertarian and think the Gov should have min influence on our lives, but just be aware that is the score. Now on the other side of the coin, an antenna mounted outside and on the back of the plane is not any good if the plane flips over and crushes it into the ground under the plane.
I personally think a cockpit mounted ELT antenna, in the vertical orintation, with a predominate portion exposed to Plexiglass and not metal structure, as far away from all metal as possible, is an OK compromise. I know the EBC ELT's have integral antennas, and the units are designed for cockpit mounting. EBC's don't have any AD's on them. There is some goodness about NOT having coaxial cable and a remote antenna to fail. If you buy a EBC unit and cockpit mount it, there is no argument, its installed per the TSO instructions. I hope EBC will make a 406 unit with like fit and function to their existing units.
I would love to see some builders join forces with a ham radio guy and get an antenna spectrum analyzer out, to evaluate some of these innovative mounting schemes. I think some set-ups will be really really poor. There are no facts just guesses. There are other threads that have beat this topic the death, so I won't repeat them again, but if you are interested they are worth searching and a read.