In Galileo, the serial ports are:
/dev/ttyS0 is the first Quark UART (UART0) and it's the Digital 0 and 1 pins
/dev/ttyS1 is the second Quark UART (UART1) and it's the audio jack one.
Your objective is to have ttyS1 in pins 2 and 3 instead of the audio jack right? I’d suggest you to take a look at /etc/systemd/system/getty.target.wants#. Here you’ll find the services that manage the serial ports:
Also, take a look at this thread Re: How to set multiple login consoles in Galileo? it has a very similar question and a suggestion that worked for the user. Hopefully you can find this helpful.
Have you made any progress trying to receive ttyS1 data in pins 2 and 3? I’ve never tried to do something like this but to do it you’d probably have to make another service that specifies that the data that is sent/received from the audio jack will now transferred to pins 2 and 3. This service will look much like the services in /etc/systemd/system/getty.target.wants#.
Try with this approach and let me know your results.