That hasn't worked properly since Acid 8 was released over 18 months ago. MAGIX doesn't care, or support their products at all. Acid 8 never worked, and now they're releasing new versions. If it's not too late, get a refund for this garbage software.
When I open up the advanced audio setting to add more audio channels, it won’t allow me to save it so no new channels are added. Acid Pro 8 is the same way, whether it’s Studio One or Live X, I haven’t figured out how to assign my existing tracks to the extra channels that show up by default either. I don’t care about midi in Rewire so that’s no problem. 😄
I installed today's update and rewire still doesn't allow me to add any additional channels to pipe audio into Studio One 4 or Live 10. Was that supposed to be fixed in this patch?
On Windows, you have to run Acid Pro 8 or 9 as administrator to add additional audio channels. And the dialog box doesn't close properly, but it does update the registry. I have successfully added audio channels to Acid Pro 8 and 9, and was able to get both working properly as a rewire slave with Cakewalk by Bandlab.
There's no point in using ACID Pro in ReWire mode, because it really doesn't do anything that Ableton or Studio One cannot do themselves, and the stock instruments/synths are garbage compared to what those DAWs bundle. The DirectX Effects, as well (the MAGIX Stuff is far more decent, but the DirectX Stuff is awful and definitely showing its age - free VSTs are far better).
I've tried such a workflow, and [IMO] it's completely pointless. It's as if I were trying to find reasons to keep using ACID Pro (or keep it installed, frankly), simply because I owned it, when it had been completely obsoleted by your other DAW purchase.
In any case, I believe REAPER supports ReWire Host and Slave, and may even support ReWire MIDI (I don't like checking, because it deletes the ReWire DLLs when you uninstall it, which pisses me off).
Having to run as Admin (or put in Admin Credentials) to get ReWire set up properly has been an issue on both Windows and macOS since the DAWn (sic.) of time.
But I question why anyone would even need to do this... What Professional DAW doesn't handle Loops well, on the eve of 2020? Even Pro Tools does this with aplomb...
EDIT: Also check and make sure you have the latest version of the ReWire DLLs. Downloading a trial of the latest version of Propellerhead Reason 10 (not 11) is the best way to ensure this, but I think ACID Pro ships older versions that a lot of other software (including Samplitude Pro, for example).
Documentation I've seen seems to indicate that ReWire MIDI is intended to work with ACID Pro. But it never has. So, I'm curious why that is.