Actually, doing both, alternating feedings between breast milk and formula is not advised. It can actually cause more problems than it solves, which include triggering serious food allergies, especially to milk.
That has been known for years (actually decades) and I have a friend whose son was the result of that.
The baby is better off just switching over completely from breastmilk to only formula, than to alternate feedings between the two.
Thanks for the info.