There’s no official Blink solution, and you’re unlikely to find out anything here about their plans, because they never comment on their road map. I did, at one time, read a post by one of the Blink staff, that basically stated they were blocked from discussing their road map by Amazon.
You can create a new account, for your third property, and both of you will be able to access it via the app on your device. The drawback to this solution is that both of you would have to log out of your existing accounts, to log into the new one, in order to monitor it and receive notifications, and of course, at this point you would not receive notifications for your existing accounts. If you only want to check up on it, once in a while, this could work, but more likely you want notifications.
There should be a way to create a clone app on your device, using one of many free app cloning apps, allowing you both to be logged in to both of your accounts at the same time, but I’ve never tried this with anything other than nox, which no longer seems to be available.
I think there’s one called parallel space, but I’m not clear about how that works.