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.
NO! Read the why not wye paper to understand why this screws with your impedances rather badly. If you need to mix em down.. you need to throw in some mixing resistors, 10k on each of L&R springs to mind...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.