Problem using 2 guitars with "Y" adapter

I am attempting to plug 2 guitars into 1 amp input, using a 1/4" "Y" adapter, specifically a Radio Shack part# 274-892 adapter. What I want to do is to keep 2 guitars plugged in, only playing 1 at a time, just for convenience to minimize plugging/unplugging.

The problem is that when 2 guitars are plugged into this adapter, neither guitar will be heard unless the volume on both guitars is turned up. Its almost like this adapter is wired in series, like Christmas tree light. Yknow, when one goes out, they all go out

The guys at RS dont know anything about it, but it seems to me that RS has this thing built wrong. I believe a Parallel adapter would be what is needed, so either guitar can be played independently of the other. How can I be assured of getting the right adapter? I know Musicians Friend sells these as well, but will I run into the same scenario? Anyone else run into this situation?