Hi,
just to be clear, when you say "2 different properties" you mean in Launch? That is fine.... the ECID is set against your ORG Id.
I have a complex setup.. technically "30 sites", but I have "core" sites, and a bunch of externals. So basically, 30 core news websites (www.domain.com), each website has an external classifieds site (classifieds.domain.com), an external obituaries site (obituaries.domain.com), etc... there are multiple offshoots....
I use a Launch property per "code base"... so the 30 core sites share 1 launch property, the classifieds for all sites share another, the obituaries have another property etc...
But the visitors moving between the core and external versions of the sites, as well as between sites are set with the proper ECID.
But, we don't have any cookie consent at this time... I suppose in your case, if each domain has it's own consent cookies, you might have inconsistencies between users who use both sites (consent on 1, not on the other... it's also possible that if they grant consent later that the ECID will be set differently?)
The best way to check this is to test... use an isolated browser, check your ECID value as you opt in and out of consent and move between the domains...