There's simpler options if all your equipment is in the same building, but yes, GPS-disciplined oscillators are a really great way to sync up clocks anywhere in the world to within a few nanoseconds. That said, it doesn't really help here because a) nanoseconds not picoseconds and b) the RTL-SDR doesn't have anywhere to plug it in.
No, it's not enough to know when samples from different SDRs were taken, they must all be taken within a very small interval (tens of picoseconds in OP's example). What the SDRs really need is an electrical signal called a "trigger" that tells them to read their sensors at that exact moment which is what GP meant by plugging them in. You can use a second GPS enable device to generate that trigger but synchronizing those triggers using time of flight is very hard.
Maybe you can bruteforce time correlation by shifting every source slightly, trying a yuge number of shift combinations and seeing which combination gives the 'best' (most correlated) output on a known signal? I've done that for many out-of-sync systems, and since I've started using GPUs in the two Os I've become obsessed with brute force methods :-)
Theoretically yes but using GPS time of flight to synchronize sensor triggers to within tens of picoseconds is so far outside of "amateur" that you might as well incorporate and start replying to DoD RFPs.
When your math starts requiring relativistic physics, it's a lot easier and cheaper to just run some fiber.