Hi BC,
The two ports on the gigaACE card are designed for redundancy purposes only. They cannot be connected to two separate devices I’m afraid.
You can consider fitting a Waves card to the SQ5, and using Waves Soundgrid for the connection between the two mixers. For example, use I/O 1-64 on the Waves card for recording / virtual soundcheck, and use I/O 65-128 for tielines to the SQ5.
Alternatively you could move the DX Hub to the SQ5, and fit an SLink card for connecting to the Avantis.