Well done to get this far, Butterfly - you've done really well; it's hard work with twins however you feed them.
I ebf'd my twin girls for a month, then introduced one bottle a month from there on - this was more for me, really, than weight gain - I had DS (21months) when they were born and it was just easier to start introducing formula so other people could feed them from time to time.
With DS I was quite a evangelical bf'er - with the girls I was more relaxed. I think with your first (or mine, at least) I was a bit PFB 'isn't he pure, nothing man-made will pass his lips'
.
By the time you're onto your second, you realise that by the time they're one, 'well-meaning' aunties and friends will have fed them the odd biscuit, sweet, crisp etc and it all seems to matter a bit less - plus I am a firm believer that a happy ff mum does more good for a baby than a stressed bf'ing mum. And just because you're introducing a bottle, it doesn't mean you have to give up on bf'ing - it's not all or nothing. Mine were mix-fed for 5 months before I dropped the last bottle.
This isn't meant to be a bf'ing vs formula post - more to say that I think you're more relaxed about the whole thing as a second time mum and realise that there is not right and wrong - I wasn't so much like that first time round.
Clearly what you do is up to you, but I know at your stage with DS, I'd have been reluctant to introduce a bottle - but second time around I was much more relaxed about how the girls were fed. That's what I'm trying to say (in a long-winded way
)