regarding why use both - I have read that they both have different outcomes - the bacteria bloom from dosing can help encourage sponge growth, the algae releases some chemicals that are good for coral and fish health - but that's not something I've specifically noticed or could prove on my own. What I have noticed is that the scrubber has a big capacity for removing stuff (for a while, my tank phosphates were lower than my make-up water phosphates until I replaced a make-up container and turned down the scrubber), but it is slower to react. Dosing is very fast to react - you can dose more or less "right now", algae growth takes a few days. So you could use them like a coarse and fine control. I run an ATS, I sometimes dose.
If you like to run things like Vibrant, an ATS may need to come off line.
regarding the OP wanting to keep both - there is an alternate installation you might consider, or you might not like. Your skimmer belongs in the sump, but you could mount the ATS above the tank. You'll get more pods in the tank possibly, but it will require another pump - so more heat and power. And visible gear - depending on how you feel about that. But probably easier maintenance access.
If you like to run things like Vibrant, an ATS may need to come off line.
regarding the OP wanting to keep both - there is an alternate installation you might consider, or you might not like. Your skimmer belongs in the sump, but you could mount the ATS above the tank. You'll get more pods in the tank possibly, but it will require another pump - so more heat and power. And visible gear - depending on how you feel about that. But probably easier maintenance access.