I mixed fed for about 6 months before they were on solids (at that point I bfd and dropped all bottles).
At six weeks half their feeds were bottles of formula, including night feeds. But having fed another baby, and had problems bfng, but then sorted them, I knew a)not to beat myself up about formula, b) that I could increase supply if I worked at it. First step was dropping the night formula. That was hardwork but it meant a) nightfeeds were much easier (not warming of bottles) b) I could feed both at once c) it gave a boost to prolactin levels
Then I basically fed them whenever I could on the breast, although I gave them bottles at fixed point in the day - say at 11am and 6pm. It was a crazy experience..
I was demanding feeding them breastmilk, but routine feeding them bottles...really odd.. I never gave up the bottles completely until we were on solids though. Perhaps I could have. I was just happy we were still breastfeeding at all, and didn't want to rock the boat..after all they had had loads of formula up till that point so getting in a state about two bottles a day didn't seem worth it.
but it did work. It only worked because I didn't have a routine for sleeping and eating. I aimed for morning nap and afternoon nap. That worked too. They did go to bed at 7.30 though on the dot! We had a bed bath routine. By six months they were in a very good routine, if that's what you want
sleeping 2 hours after lunch, and regular bedtime, waking once or twice in night only, waking at 7.30/8.
By the time they were six months and breastfeeding out and about, it was a complete joy not to have to prepare formula and cart bottles around, and to have the pleasure of snuggling up for a feed, early mornign feed cuddled in bed, comfort them if they woke at night, felt a bit under the weather. No problems with d & v, ear infections either. Fed them till they were 2 years.
Go for it if you possibly can, mixed feeding can work if you decrease the amount of bottles they get, and work on the demand side of it. Also make yourself very comfortable when you feed, loads of pillows, tea, radio, phone, so that it is a pleasure not a torture. I also allocated one breast per baby - some think swopping is better. My other trick but I'm not sure how you would feel about that or how much help you have, was to go out with one baby for the day, and just concentrate on feeding that baby on both sides, build up a bond etc. That certainly upped supply too, even if the other baby was bottlefeeding the whole day. Then the other baby got the benefit of the upped supply etc etc.