So I have two Sylesti's in my stable, how do I switch which one battles if that's possible, and is it possible to use both in a battle?(the one I cant switch to is a lv 8 and im trying to get it to lv12 with the other one, I've only played for like a day so)

If you go to the sylesti plaza in Esior's Haven and scroll down, you can buy the first party expansion (the second party expansion is obtainable from the plaza in Sanctuary of Saerielle, in the second region, once you progress the story that far)
(You may need to grind gold via Missions or battle first)

Once you get the slot, you can go to the Party page and add the new pet to the party. (You can swap pets that are in the party into different positions by clicking one then the other. You can remove a pet from the party using the red - in their party image's top right corner)
You then have to click the "Set Party" button.

thank you