Update: first test looks promising.
Mautic login and backend works no matter if I call it through http://ma.domain-a.com/ or http://ma.domain-b.com/. Also landingpages can be reached through both ways. This will probably be the same for the scripts to embed and the forms (will test next).
I could image, that automatically replacing links in email will hold the original URL (as Mautic system doesn’t know about this dual URL setup). Not sure, if this may influence spam score, if links differentiate from sender URL?
Also system pages will always come from the original Mautic URL and theme. So they need to be designed in a way people understand its still the same company.