Connecting bolth audio outputs to one sterio cable is not a good idea. It can cause unwanted signals to be forced back into the outputs. Your only inexspencive option if you want to be able to hear both at the same time is to get computer speaker system that has two inputs and use them as they will alow you to hear both sterio signals at the same time. Altec Lancing has several that do and so do Yamaha and some of the higher end Logitec speaker systems. You can sometimes get small mixers on ebay that would also do a great job like .
this or
This oneand also
This one