one device with two power supplies connected different voltage outlets
How well or poorly this works is mostly subject to how well or poorly the power supplies in the device are engineered, and how well or poorly it actually "hot-swaps" in practice.

In the fluffy bunny sunshine &amp; flowers version, the fact that you have individual AC to DC power supplies, that are independent and which supply, share, and should be able to hot-swap the DC load means "no problemo, dude." It could go that way. 

DEPENDING on it going that way is a whole 'nother ball of wax. 

The AC side of the devices should never be getting shared or come into contact (at least with typical hot-swappable dual supplies, each having it's own line cord.) I'd want to be VERY sure the grounding was correct on the 240V UPS, though - would not want fighting grounds. Mostly a concern from it being portable, and wondering where it's plugging in, .vs. where your 120 supply is plugging in, and if the 240V extension cord that presumably takes is wired properly. Again, it SHOULD be fine, if everything is wired properly.

More broadly, any time one stresses items which are supposed to prevent failure (ie, hot-swapping supplies) that have not been regularly tested to verify that they do that correctly, you do have some potential that you will find that they don't do that correctly, and you'll crash whatever it is that you so desperately want to keep up and running. Of course, the potential for crashing under testing keeps folks from testing the functions much, if ever - a nice catch-22.