Thank you for reaching out.
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.
Thanks.