Not very sure I completely got your question. But from what I gathered, if the first level approver is also one of the second level approvers, then he/she will not be part of that list of second level approvers because he/she has already approved at the first level.

Assuming you're in flow designer, I would probably use an if condition here, where we will validate if the line manager is one of the users in the second approval group. If he/she is, then I will include logic to omit that user from the second approval group.