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.