Is it safe to mix baby formula? (similac sensitive with enfamil infant)

Depends: Formula is not paperclips. There can be minor differences in the ingredients that babies gut may deal with differently.This can result in changes in digestive patterns, cramping,loose stools,constipation etc.while switching back and forth. That said, some kids won't notice any difference and will do well,(or poorly) on one or both. Best find one that works and stick with it.

The mixture of formulas may result in different proportions of vital ingredients. If nursing is not an option, select a standard baby formula as tolerated. Watch for signs of rash, diarrhea and vomiting. If there is formula intolerance, also exclude milk and soy allergy.