As for cross-talk between antennas, I'm not sure this setup removes it
completely; there's probably leakage around connectors, and possibly in
the cables attaching the outwards-facing SMA connectors on each machine
to whatever the tiny connectors on the cards are called. Don't have the
equipment to measure any of that, though, so for now it remains a hunch...

Yeah, that's going to depend on the noise-floor of the receivers. I've found that -105dBm receive radios (other technologies) needed physical separation and cables rated at 60dB of shielding in order to keep cross-talk down.

The cables from the SMA connectors to the boards are likely pretty leaky (the board connectors are probably u.fl connectors)