LeftAux fuel first, then Left and Right at the same time

Have been struggling with this one, maybe anyone has got an answer.

For the Savage Carbon I am trying to add a LeftAux tank.

However, this plane (a STOL mod based on the Savage Cub) has no fuel selector. How do I get this setup such that it feeds from LeftAux first, and when empty starts using Left and Right concurrently?

The other way around is also acceptable…Left/Right concurrently till empty, then use LeftAux.

Thanks in advance!