In my experience, if I open up Chrome Incognito (so there are absolutely no cookies available), and I open Site A (domainA.com) in one tab, then open Site B (domainB.com) in a another tab, both sites correctly stitch me to the same ECID without needing to do any special effort.
However, that said, Chrome does still accept third party cookies... I know that this is limited on some other browsers...
If you are expecting that a lot of people will click through from Site A to Site B, then you can use try using https://experienceleague.adobe.com/docs/id-service/using/id-service-api/methods/appendvisitorid.html?lang=en and append the visitor Id, then read the value on the other site.
But if the user isn't going direct from A to B, I think you just have to rely on Adobe to do its best to stitch the user properly.