2 VHFs sharing 1 VHF antenna

I have an Apelco VHF mounted in the nav station with a remote speaker in
the cockpit. A Raymarine handheld is kept near the helm. Of course the
handheld has nowhere near the range of the fixed VHF below. Instead of
replacing the [perfectly good] Apleco VHF, I would like to install a
second fixed VHF at the helm connected in such a way to share the mast
top antenna.

Is there an adapter or junction/splitter type device which will allow
both VHFs to be on at the same time (not transmitting, of course) and
share a common antenna without manual switching or other intervention?


--
kailaniskipper