Thanks for your replies. They'd help me come to the conclusion that it might be ok to start by taking away one feed - and then we all got the most horrendous cold/throat/cough thing and the only thing DD1 would have for two days was... milk.
Looks like I need to carry on for a bit and try and fatten her up in the meantime. Not sure how though!
If you are still feeding DD2, personally I would carry on. I know it's a bit of a pain, but with her being so low on percentile, it might be a big reassurance if she gets poorly to still be able to get milk down her, and also to know she's getting that extra healthy supplement.
Also (though your DD might not mind) but is there any chance she'd resent her sister thinking she's "taken her mik" in a sense if she stops feeding but her sister still is?
Though if you really are finding it hard/she takes absolutely ages having a leisurely feed it might be best to stop. Or give her a time limit maybe - say a 5 minute feed each morning and evening?
Hm. I stopped feeding DD at 31 months as I was experiencing strong "nursing agitation", being pregnant. Actually, my milk dried up at 17 weeks and DD comfort nursed for about a week which was very uncomfortable, and when she stopped persevering, I didn't push it. I had always envisaged tandem feeding, but just couldn't get through the aversion bit.
I really hear that you want to do all you can for DD1 who I am sure is benefitting lots from your milk, and I can see how you may think that you perhaps 'ought to' continue, seeing as you'll be nursing DD2 anyway.
However, I was surprised at how easily DD let go of bfing, and that what had meant so much to me seemed of much less consequence to her.
If you feel that the time has come, you may be surprised at how easy it is to stop.
My lovely DD is now 26 months old and still having two feeds per day - one on waking and one before bed. I also have a 16 week old who is ebf. Before the new baby arrived, DD1 was down to just the evening feed, but started wanting mornings again when DD2 arrived and I didn't feel I could say no, with all the upheaval.
I had increasingly bad nursing anxiety while I was pregnant and really wanted to stop, but we hung on in there and now it's all pretty much ok again. However, I'm returning to work in a few weeks and I really need all the time in the mornings that I can get, particularly as I'm going to need to express and that's the best time for me to do it. I could also really do with dropping the night feed - again for time and flexibility reasons.
However, while the new baby is on the 98th centile and as fat as butter, DD1 is, and has been since about 4 months, on the 2nd centile - she is actually not very much heavier than her little sister. She's also at nursery and picks up all the usual vast array of bugs, plus she does the usual toddler thing of eating like a horse for a few days and then nothing but toast for a day or two. I'm worried she has no spare reserves and that BF is still providing her with a really good source of calories, and that removing it would be massively selfish of me.
So I would welcome your thoughts: she is a PFB of course; so am I worrying too much about this? Milk supply is massive so am not worried about tandem feeding effects on DD1.