Can mono RCA split to L/R stereo RCA?

Probably a very basic question, but one has been troubling me as of late...

I have a surround sound center speaker cable which I would like to route to a speaker that takes L/R inputs. If I use a simply RCA Y-adapter I get sound out of the left speaker only. I would like both speakers to push the same sound, albeit a false stereo impersonation. Can this be done? Is it just that I have the wrong connector?

The equipment is pretty straight forward. I currently have a Bose AcoustiMass 6 system setup with the center cube speaker under my Pioneer PDP-5070 with its add-on stereo speaker at the bottom. My wife hates seeing the center cube speaker, so I have to detach it/hide it whenever it is not in use. I was thinking that if I can route both the sound of the TV and the center channel speaker of the surround sound system to the same Pioneer speaker. then I would have a cleaner setup.

I have been able to connect the center speaker cable to either the Left or the Right RCA inputs of the Pioneer TV speaker with full functionality in that selected speaker, however I would love to have the audio come out of both speakers., so I was thinking that if I could have the center channel sound double to a Left channel/Right channel setup, then I can connect both the TV and the doubled center channel speaker to an audio switcher, with a L/R cable going into the Pioneer TV speaker.

I used a Y-connector and it worked fine for stereo feeds. However it didn't work with a 5.1 signal, as the center dropped out. I think it has to do with the way the speaker setup handles 5.1 as the subwoofer unit does the decoding and routing.