In theory if you lose 1 line the miner should simply shut down but I would not bet on that happening before the PSU is damaged...
I have nowhere near the experience you do when it comes to electricity and all that, but I think that this will work just fine, I "unintentionally" had a few miners hooked up this way and nothing went wrong, in fact, the moment the PSU loses one of the two inputs it will
instantly shutdown, it will only keep the PSU fan spinning if the other input is plugged in.
I believe as long as the input voltage is the same (which should be in OP's case) it should work just fine.
I think the APW9 PSU is just two independent PSUs combined, however, since this is your field, check it out
https://www.zeusbtc.com/manuals/repair-guide/Antminer-APW9-power-supply-repair-guide.pdf, you might be able to confirm if there is an actual risk supplying it from two different circuits.