Using a Second 2GIG-GC3-345 as a Secondary Station

Can you use a second 2GIG-GC3-345 as a secondary controller (similar to using a 2GIG-SP1-GC3)?

No, you can only use one GC3.

The SP1 is more or less functionally identical to the GC3 and uses the same hardware case and same firmware. It just does not handle programming or communications to sensors/cell.