Say, you want to transfer the website
example.com from account
B. There is no direct support for this but you can achieve it by cloning the website 2x.
Mark the website
example.com as open source in its branch settings. This is needed so that account
B will be able to clone it.
B shall clone the website
example.com under a temporary domain, for example
example-clone.boomla.net. I recommend you use a
boomla.net subdomain as then you will have instant access to it (no DNS setup needed, no need to wait for issuing a HTTPS certificate).
Visit the cloned website, verify it works and that you cloned the correct website.
Delete the original website
example.com on account
A so that you will be able to create a new website on the domain.
B shall now clone the website
example-clone.boomla.net under the desired domain
example.com. (No need to mark it open source this time as it's owned by the same account.)
Again, visit the cloned website, verify it works and that you cloned the correct website.
Delete the temporary website