It is technically possible to add the same account as a Master and as a Slave at the same time.
However, to do so, you need 2 distinct Trade Copier cockpits (2 distinct usernames). When you try to add the second account, you may get an error message asking to contact support. In that case, we just need to manually allow your second account. Please just ask if it happens. Please note that we do not allow this with Free subscriptions.