You'll want to look at the KrakenSDR. It's basically a circuit board integrating 5 RTL-SDR chips driven from a single clock source and calibrated for phase coherency, for the express purpose of doing radio interferometry amongst other things. They're in stock at Mouser.
Whoa yeah this looks like it would be easier than doing it all by hand. $466 at Mouser. Interesting!
Looking around on Google, I don’t see anyone that has tried using this for interferometry yet but the KrakenSDR team explicitly mentions that it can be used for interferometry.
They were pitching it for use in bistatic passive radar, which is a very similar application, until somebody put the fear of ITAR into them. Some of that code is probably still lying around.
