The switch has now been tested with separate antennas feeding into each of the inputs, and the port-to-port leakage is minimal to non-existant. So the isolation is good enough.

If I connect an antenna to one port on the left-side of the switch, and leave the other left-side port unconnected, then the antenna's signal does leak over to the unconnected port. Much weaker, but sometimes viewable on screen. Ditto for the right-side pair.

But in practice, with antennas connected all around, there's no visible interference between ports. Woo-hoo!

Cheers