yep i bfed ds2 and ds3, and then ds3 and ds4 together.
i tandem fed ds2 and ds3 for over a year and it was fine, ds2 gradually cut down during my pregnancy, in the first trimester i stopped his night feeds as i was knackered.
then gradually he cut down to three or four feeds a day which is what he had once ds3 arrived.
i didnt find it changed his poo that much tbh, maybe it was a bit splattier, but he was eating three meals a day etc so i didnt notice a huge difference in his poo.
your body will make as much milk as a baby or babies demand so he wont use up all the milk.
the littlest one slept in so he kind of had first dibs, but then ds2 would come and join up and i would lie in bed and feed them both, the baby in the crook of my arm and ds2 snuggled up on the otherside, i used pillows underneath the arm with baby to bring him higher etc, i have largish boobs so they kind of flop over to the side...
tbh having a feeding toddler is great in the first few weeks for relieving ingorgment etc.
i just had the attitude that if he asked i would let him have it, but generally he stuck to about three feeds a day once ds3 was born, morning, afternoon and bedtime, i always made dp look after baby for the bedtime feed so that i could give ds2 a lovely snuggly lie down feed etc and he could have some 'mummy time'
tandem feeding wasnt something i planned on doing but it worked very well and i had no problems and it actually meant i could get ds2 an dds3 to sleep at the same time and we could all have a nap