Mixing formulas

Audrey • I`m 31. I`ve been married for about a year and a half. Started TTC Oct 2014. Can`t wait to get my BFP!!!
My baby is 2 1/2 weeks old, and since day one has been gassy and spitting up a lot. He was on regular similac in hospital so I continued him on it until my pediatrician recommended Enfamil AR. We switched and he became very constipated on AR. I now mix half similac and half AR and it seems to help sort of. Anyone else experience this issue and mix as well?