Just found an rca to left-right 1/4" jack at guitar center that will allow you to use two channels of your mixer. I don't know if I read the question correctly, but I'll send a link as soon as I find it on their website.

Edit; was easier to find then I thought.
http://m.guitarcenter.com/Item/Default. … &urx=1
Again, not sure if this is what you're looking for, but just found out about this a couple days ago and will be using it myself.

You can't.
Since RCA only have one signal point and one point for ground it's impossible to send stereo (two signals) with one RCA..

You could send the signal from the RCA to the left and the right of the stereo jack, but it'll still be mono.

As I'm not very advanced in electronics and held a soldering iron only when I had to fix torn wires, I think i should ask a question there.

Game boy outputs signal to 2 channels: left and right. If I had an RCA prosound and a jack-input mixer, how do i make a single rca-out be a stereo signal? Will just leaving the tip/ring disconnected be fine?

Thanks.