The ISS is a sufficiently large mass of metal that tremendous same-band isolation should be achieveable by simply mounting antennas on opposite sides of the station,...[etc]
Unfortunately, Not really. I am sure that if we had two magmounts and could crawl all over the outside of ISS with long enough cables, that two mutually isolated locations could be found (kind-of-a- can you hear me now? Kind of test). But NASA does not allow us the resources for that kind of approach.
But even if you did find such a location, or even if you did spend a few millions of dollars doing an RF analysis of such an arrangement, it would all change when ever anything moved. And things are always moving up there.
Also, not only does one not even attempt to plan weak signal same-band operations from the same Field Day site, one would never risk several years of planning and millions of dollars of effort on something that critical that might be desensed as soon as the solar arrays moved 10 degrees..
We had several amateurs do tests of several 2 meter radios to discover how much power on one 2m radio was needed to descense another 2m radio and the power level was down around 10 milliwatts at 100 foot separation and it still caused 10 dB desense. (I think I remembered those right)...
Unfortunaly then, we simply cannot plan on up and downlinks in the same band while hoping for independent operations without constant crew intervention with every single mode change that affects that band.
Bob Bruninga