HI, I am new to this forum...
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For your problem, I think you are using the wrong Y-adapter:
http://www.radioshack.com/product/index.js...rentPage=family
This one is mono-to-mono adapter.
Actually, you need one dual mono to stereo adapter.
Ref:
http://www.emu.com/products/product.asp?product=13342
But, notes: this one is 1/8" jack for stereo output...
Hope help you...