Can my partner in business B see business A?

On two separate accounts: no, by design — nothing from one account is visible from the other, not even the existence of the lines. On one multi-line account, roles apply to the whole: an admin or operator sees both businesses, and so does read-only, unless restricted per line. If you want partial cross-access — view A without touching B — the clean setup is two accounts with a read-only view at head office. Describe your structure to us, we'll configure it.