I just wanted to add my bit. I have been breastfeeding my twin sons for almost 4 yrs (yes I am crazy) the last year and half it has only been twice a day or when sick or after being hurt etc.
I am very ready to wean them but they aren't so keen lol. One twin didn't start BF till he was 5 months old so I pumped for him for those 5 months.
Once they came home at 20 days old I was fortunate I had a good supply so I never had to give them formula.
I endorse all the advice as previous people said.

I was a bit disappointed at first that so many people give negative advice regarding how difficult they think BF twins will be.
Even the Dr's and some midwives were pushing 'formula' via bottle because it would be much easier to get them into a routine and help them grow quicker .

I had to struggle to get alternate tube/breastfeeds happening ...they kept saying BFing was tiring my DS1 2.9kg , though he was 1 month premmie boy .DS2, his brother 2.1kg ...incidently refused to BF because they kept shoving a bottle (with EBM) in his mouth !
DS1 kept refusing the EBM via bottle overnight so they said he was having feeding problems, put feeding tube back in and made us stay longer grrrr. I was there from about 7am to 11pm and he was he BFing fine.

In the end I agreed to give offer them both a formula bottle after every BF ...just so we could go home from special care nursery. Of course we just never had time to go shopping for formula what being busy with twins & all.
I knew they were fine lots of wees & poos (OMG double trouble)
It isn't easy from the get-go having two tiny babies to care for let alone mastering BFing and if they are premmie even more so. But it is doable with the right support.

A friend of mine was in a pre natal class and the instructor made a joke of holding both the dolls/baby dummies under her arms in a football hold and had the class laughing at how funny it would be...of course the friend (the only twin mum) decided then & there she wouldn't even bother trying. Maybe she had other reasons but it was unnecessary.