You can also find some passive summing devices such as these, STD-10K Passive Audio Divider/Combiner - 10 kΩ, or an active device that provides some isolation and input level control such as this, ST-MX3 3 Channel Audio Mixer - Line input and output.
After loosing our (only, yes I know we need more than one) 3.5m Mini-Phono to XLR lead, I had to make one up, rather quickly, I got a 3.5m MiniPhono to Sterio RCA, and then used a video splitter (just a one into two, nothing like an amp) I just used it in reverse, so I plugged the two RCA's in, and a RCA to XLR on the other end, this worked fine for me, it will work with your red & white RCA's.
Hope this helped, and I didn't blow anything up.
Nick
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