I think my daughter is a breastfeeding snacker. Is that even possible?

I'm a FTM and didn't know to offer both breasts during feeding which caused her to lose weight. Now I offer both and she's gaining weight. I do 20 min n both sides, but I noticed she's throwing up more. If I do 15 min on both sides will she be getting enough fatty milk?? Will it diminish my milk supply? Is there any one else who's baby's are more snackers then meal babies.