[semi-extrinsic]

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.

[thoop]

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.

[CamperBob2]

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.

[jesprenj]

I mirrored whatever they had when they got taken down on github: http://ni.4a.si/anonymous/krakensdr_pr/

The wiki is still public afaik.