I didn't post that link because I wasn't sure it counted as a genuine ABY pedal.
As it happens, it seems this is what Stew is looking for, and it was what I needed when I bought one, but it's actually an instrument selector ... it does 2 x instruments into 1 x amp.
I now need (occasionally) to do 1 x instrument into 2 x amps, so I've bought a cheap Fender ABY (unbuffered, no level controls) which can do either 1 --> 2 OR 2 --> 1 depending on my needs.