Congratulations on your twins! Feeding preemie twins is HARD, mine are 14 moneths now and I shudder looking back at how tough those early days were!
You have options. You should do whatever works best for you and give any doubts the finger. I guarantee however you feed them, nobody will be able to tell in the long run.
I did combi feeding. One twin in particular was struggling to latch and was losing weight as soon as he was off his nasogastric tube. What worked for us in the early days was I'd chage them before a feed to make sure they were wide awake, spend 5 mins trying to feed them so they were hungry and working hard. That way I figured they'd get antibodies etc. Then if give them their bottle (same amount as they were having when tube fed). Afterwards I'd offer the breast and they'd generally have a relaxed comfort feed and nod off. Until they were past their due as I pumped for the bottle, then I ing some formula. It was recomended to me to mix formula with expressed milk as it contains enzymes that make the formula easier to digest, but I only had to pump half as much.
We kept that up as a routine for about 3 months or so. At that stage they were much stronger and found latching easier, so I ditched night time bottles as I hated the faff. I knew they still had bottles during the day so I wasn't worried about them starving, but the transiton was totally straightforward. A month and a half later I ditched all except for one daytime bottle (which was total formula, so no more pumping). They stayed on that routine until the ditched the last bottle (their choice, not mine!)a couple of weeks ago. They still breastfeed when they fancy it, but mostly they drink from a cup.
For me, breastfeeding was easier - I never liked bottles. So if you combi feed now, up can always gradually up the breastfeeding IF that's what you want. Or if breastfeeding is too tricky, then you can move to bottles (expressed or formula, whatever works for you).
Combi feeding was very convenient for me in the early days. If you want to do it, you'll be fine. Please don't feel guilty whatever chi you make.
Happy to answer any logistical questions you may have. Twins are hard work in the early days!