Feeding

In a FTM of 1 month old twins. I have been breastfeeding, pumping, and formula feeding my babies. At night I usually just pump and formula feed. After the babies finish their bottle, they sometimes seem to want more. Is it okay if I pour some of my breast milk into the same, empty bottle they just had formula out of? Or should I use a completely new and clean bottle for the 1 oz of breast milk I use to top them off? Thank you!