just my opinion, for me it has no sense to have restriction in usernames in multiple companies, if i'm user of company 1 with username "blesta" or "paul<atttt>blesta.com", and i want to purchase from another company i will make my habitual usernames "blesta" or "paul<attttt>blesta.com", seeing this already used is something that make me confused.
i'm as a user company A is not company B . and it has no sense for me if in my account in company A seeing or i'm impacted by something else in Company B.
i'm as CEO or owner i don't need to reveal that i have 2 or 3 companies has the same database or they are related.
normally and logical company A is not Company B, and blesta should take care of some issues like those, or the multi-company has no sense or no advantage at all .