Hi S.,
Congratulations and kudos for mothering twins and for wanting to breastfeed them!! This is absolutely possible--I have known quite a few moms of "multiples" who did this successfully. I do understand that feeling of overwhelm when you consider all you must do as a mom of twins--ANY mom feels that, but when you have two babies at once, it's probably exponential! The thing to do with that, though, is let go of your concept of who else you are "supposed" to be. Right now, you are a Mother! You are a fully working, mother of two new humans who need everything you've got for awhile in order to start their lives well. AND...You can do it! Whatever your spiritual basis is, please believe that nothing is given to us that we cannot handle, and it is an honor showing your strength and ability to have been given two little ones at once! What a special thing this will be as they grow and as you grow with them!
As far as switching them completely to breastfeeding, you are right to do so--don't let anyone talk you out of it. It will be not only the best way for them to be nourished, but you will find that--even though it is very challenging right now as you all figure out life together--breastfeeding is by far the simplest, by far the easiest, and by far the most useful method of feeding (and comforting, and consoling, and getting them to quiet down, and sweet, and loving, and, and, and).
Now, the hard part...I'm sure you had good reasons to begin their lives with part formula from a bottle and part breastmilk, but when you do this, babies tend to get "lazy" about breastfeeding. It involves more work. A bottle does not take much sucking effort at all, whereas breastfeeding involves correct mouth placement, working ALL the facial muscles (which is very good for them and helps them develop their mouths and facial muscles for speech and for tooth placement), and sucking hard until the "letdown" occurs--at which point you may have milk literally squirting out, or you may not but it is not as much work.
Most lactation consultants and midwives recommend that breastfed infants learn to breastfeed very well, and for a good long time to develop the correct patterns, for at least 6 weeks or more before giving them a bottle. Since yours had the bottle first, it's a little like trying to convince someone to read the book when they first watched the two hour movie.
But it CAN be done! I suggest finding and working with a lactation consultant, who can watch you breastfeed your little guys, and assess how they are latching on--giving you specific help with getting them to move away from the bottle to breastfeeding exclusively (though you may want to keep the bottle as a once a day option, so Dad can get in on the snuggles and help you on those days when you just can't seem to get the babies out of your arms!).
Yes, it is feasible. Yes, you can do it, and your babies will be GREAT breastfeeders--once you get them past this hump. But definitely get some local help. La Leche League offers free help, and can also put you in touch with more in depth consultants who may charge a fee (it's well worth it!) if you need it.
Fiora