In order to take data from one tenant and save it in another, you'll need to have accounts in both tenants. When you want to read from/write to tenant A, create and use a connection using the credentials in tenant A. Likewise for tenant B. So, you'll have actions in your Flow that use connections for both tenant A and tenant B. The only licensing implications are that you'll need user accounts in both tenants. That said, there are security implications that you may want to consider.
