When a baby does this it can possibly be:
1)no let-down or slow let-down
2)taste of milk changes for some reason (maybe what you are eating)
3)teething
4) poor latch
5) Thrush... if in their mouth they have 'white patches' this can be oral Thrush and it makes it uncomfortable for a baby to nurse
6) your supply
7) illness, they are sick... etc.
8) nipple confusion... since they are used to having a bottle during the day for the majority of the time, maybe when they are put to breast they are not used to the nursing... because breast-nursing and bottle feeding takes 2 different 'techniques' for 'sucking.' The bottle is just by gravity....and breastfeeding takes a whole slew of mouth/tongue/throat coordination of the muscles just to get the milk flowing... and sometimes a baby gets 'impatient' with breastfeeding as a result.
Next, are they just rejecting breast-nursing... or are they rejecting the Formula bottle too?
Do you nurse them at night once you get home from work? AND on demand? They should still be nursing on demand, once you are home with them. Direct nursing, when you are available, is still the best way to ensure your milk supply.
Next, do you exercise? Sometimes, with an exercising Mom, exercise produces lactic acids in the breastmilk, and makes it taste 'soured.' Therefore, nursing has to take place before exercise... or after-wards it can take a couple of hours for the milk to return to 'normal' taste. A baby will often 'seem' to reject nursing, if they are nursed directly after exercise, because of the change in taste.
And yes, see a Lactation Specialist.
Don't assume they are self-weaning until you can figure out what may be making them 'seem' to reject nursing. But of course in the meantime, offer the breast... but if they continue to reject it, you DO need to feed them... even if that means their supplemental Formula.
For the 1st year of life, breastmilk/formula is the PRIMARY source of nutrition for a baby.... NOT solids, NOT water, NOT juice. So, you must give them something... even if only the Formula.
Are you still pumping a lot at work? Still keep that up as much as possible. Are you keeping hydrated? Eating well balanced? Do you have stress? Sometimes stress can affect a woman's milk production too.
How much is coming out via pumping? Or is your supply still waning?
Each baby is different, but both your twins are doing the same thing, in response to being at the breast. So that is interesting.
For me, my son self-weaned at about 1 year old... and after that he just wanted the bottle. I was giving him a bottle of Formula as a 'chaser' to my breastfeeding him... because he would suck me dry, on both breasts and would STILL want to feed. So per our Pediatrician, I gave him supplemental Formula... but only sometimes. But, for him, the bottle was just quicker and easier, since it flows only via gravity... they don't have to 'work' at it to get the milk. Then at about 1 year old he would literally slap my breasts away and turn his head, grumble/scream and indicate to me he just wanted the bottle.. and he was done with breastfeeding. So that was that.
But for your twins... I would ask a Lactation Specialist first... and then rule things out, to make sure first... BEFORE you 'assume' your twins are self-weaning. THAT would be your first thing to trouble-shoot this dilemma. But make sure they are getting enough intake in the meantime... very important.
All the best,
Susan