Should work although maybe with double splitting there's a chance of interference or added resistance or something that'd downgrade the quality? (I'm no electronics expert.. just wondered if all those splittings will cause some buzzing if the gear isn't top notch).
I bought myself a fairly cheap mixer.. like 30 bucks or so. has 2 mic inputs that can be used with phono jacks as well, and a couple of stereo phono channels and RCA as well. That way I can patch in my mp3/phone, or a buddy's guitar, or keyboard, and mix the volumes separately.. seems if you're doing it only at the module you'll have to be careful that one instrument doesn't blow away the others.. I imagine you can just keep the volume in check at the instrument itself, but (again, I don't know all electronics) I feel like there's times when an input brings the feed in without volume control and you may get a blasting if you can't easily turn up or down the volume of the individual instruments.
I think a mixer would be safer.. plus, they're an awesome device to have around.. one day I felt like plugging in a mic and singing along and voila, easy to do with the mixer in there.