Lets that, connecting everything works.. kinda.. or not.. kinda, confusing? I'll explain.
First, I assume your sterio (

) has only one input.
Second, sound mixing, on a mixer, is really, connecting the wires together from all sources. (not only, but is basic idea)
However, mixers have coupling capacitors and resistors (RC grid) to balance the overall resistance and noise cancellation.
You should use a mixer to connect both sources to the "sterio" 2 input mixers with one output come in very cheap like this
cgi.ebay.de/OMNITRONIC-LH-025-3-Kanal-St...ht_3377wt_1165" onclick="window.open(this.href);return false;
If any more doubts (the isn't much more to say about what you want) drop a post here