I was playing around with the rtl_sdr dongles and came up with a trivial hack to build a receiver with multiple coherent channels. I do this basically by unsoldering the quartz clock on the slave units and cable the clock from the master rtl dongle to the slave units (I've attached some pictures).
You still have to do sample alignment in software, but this is relatively easy. There are a lot of cool applications, such as a dual frequency beacon satellite receiver, interferometry, or passive radar that you can now do with $16.
rtl1.jpg
rtl0.jpg