Although you can use splitters for this kind of job, you can run into problems using them to combine two inputs into one set of speakers. I used to do this and found that the volume is much reduced on both - if I remembered my electronics knowledge from university better I could explain something to do with input/output impedance of amplifiers. Anyhow, connecting the outputs of two together kills the signal a fair bit, and this always struck me as a "bad idea". That power is being dissipated somewhere other than the speakers, which logically must be the output stages of the other sound source, and I'm sure it's not designed with that in mind.
I have a similar setup with my 360 and PC into the same monitor and set of speakers. I plug the sound output of the console into my soundcard's line-in, and tweak the volume on the PC accordingly for good balance. You need the PC on for sound, but a lot of folks PC's are on most of the time anyhow.