There's no restriction wherein you're locked out after logging into a single YouTube account. The TubeBuddy apps let you log in for free, to end numbers of your YouTube accounts as long as you have an account with TubeBuddy, regardless of whether it's a starter license or the paid one and you're logging through the same Email/Gmail id when it comes to accessing your channel. Please do note that, If you have multiple Youtube channels under one single Gmail account, which is the case when it comes to brand accounts you will be able to see all the channels in one login attempt, provided, you have made a TubeBuddy account for other YouTube accounts.
Since this is already a feature or a perk which we have for our TubeBuddy Application, I don't see it fit, but If you have any queries or questions, you can reply the same. We will be closing this post in next 24 hours.