If you're planning to switch your daughter to the other brand of formula permanently, then yes, you can mix them together to get her used to the new formula (I'll explain how in a moment). But if you're using the other brand as a stop-gap while you get more enfamil, it's really not a good idea. Different formulas have slightly different ingredients, and babies - newborns especially - have very sensitive stomachs when it comes to new foods. This is why you don't give babies under 4-6 months anything BUT breastmilk or formula. Their systems are just not ready to process different types of food. If you're going to switch your daughter from one brand of formula to another, this is how you should do it. Brand 1 is the old formula; Brand 2 is the new formula: Day One: For every feeding, use 3/4 Brand 1 and 1/4 Brand 2, mixed together. So if your daughter is drinking 4 ounces of formula at a time, you'd mix together 3 ounces of Brand 1 and 1 ounce of Brand 2. Day Two: For every feeding, use 1/2 Brand 1 and 1/2 Brand 2, mixed together. (For four ounces total: 2 ounces Brand 1, 2 ounces Brand 2.) Day Three: For every feeding, use 1/4 Brand 1, and 3/4 Brand 2. Day Four: Use only Brand 2. What I found helpful when I was switching my son from Similac to Enfamil was to premix the day's formula and then pour out exactly what I needed. (Pre-mixed formula keeps for up to 24/48 hours in the refrigerator.) You'll find that her poops may change consistency and frequency during the switch, as well as for a short time afterwards. Don't worry about it too much, unless she's not digesting the new formula or flat-out refusing it (which can happen). You may need to stick to the Enfamil if it's the only thing her tummy can take. If cost is a problem, most of the big-name formula makers have programs where they send you coupons regularly to help defray the cost - and for me, I know I found fantastic deals on Enfamil at Costco (often half the price as at the grocery store). Good luck!