Mixing Formula Milk and Breast Milk

I have a newborn. This is my first baby. 1 month and 4 days na age niya. But I delivered him when I was in my 35th week. As of now, medyo mahina supply ng milk ko. Advised naman ni pedia na pwedeng mag formula at the same time mag breast feed. Ginagawa niyo din ba ito? Ako kasi kung formula siya ngayon, sa next feeding niya breast milk naman. Thanks ?