Hi and welcome to this forum

Yes, you can use the 16.4.2 as a submixer into the two ioStation inputs. It will not give you more channels into your computer but you can leave everything connected and use the mixer to choose/mix what you want to record.

It also puts an extra A/D and D/A conversion between the microphone and the computer. Do a few tests to determine if that affects audio quality more than you would like. Make sure you keep levels into the 16.4.2 between half way and 3/4 on the meters. The 16.4.2 doesn't like hot input signals

Note that the 16.4.2 has no instrument inputs. Best to use a DI with the guitar.

Hey. Thanks for your reply. No sure what you mean that 16.4.2 has no instrument inputs?? It has 16 at least. Right?

It has line inputs and microphone inputs but it has no high impedance instrument inputs as you would need with passive guitar elements. The remedy is a DI box between guitar and mic input.