A couple of thoughts... as others have said, changing things right before you go on vacation is always a bad idea. It sounds like you do need the ATO (unless you can have someone come by every day or two to check the tank and top off manually, which might even be better) so what I would do is get the ATO set up immediately... as in right now, so at least you can monitor it for a few days days before you go on vacation.
You don’t want to be adjusting it every day and trying to fine tune it over the next week. You want it set up to the point where it is functional, even if it’s not perfect, and you can leave it without touching it for the next few days so that you will have confidence that it will continue to work the same way while you are on vacation. If you need to adjust it or tune it every day then how do you know that it will be good when you’re gone?
After you come back from vacation you can set up the overflow and the sump and move the ATO to the sump and get everything fine tuned there. As far as your original question, I do run two return pumps in my sump, which are connected to outlets on two different circuits. I also have two heaters, again connected to two different circuits. About a year and a half ago I had a case where the GFCI on one of the circuits tripped while I was on vacation, but one pump and one heater kept operating and the tank was fine for several days before I was able to get back and correct the issue.
But like I said, now is not the time to be making big changes like that... get your ATO set up immediately so you can monitor it for a few days before you leave and make the big changes after you come home.