Is there a reason you can't just double the amount of chicken while keeping sauce ingredient amounts the same??

I wouldn't be terribly concerned about the chicken fat issue. This particular recipe calls for boneless skinless thighs and most of the fat is in the skin.

Assuming your slow cooker can accommodate the extra volume it should be fine.

If you have an instant pot that is large enough to hold all that chicken, then I don't see any reason why you couldn't double the meat amounts. The only thing I'd consider is the extra amount of fat that double the number of thighs could release.