no you still make colostrum from mid pregnancy. some kids wean because of this.(the bigger kid LOL).
if the elder still wants to bf after the birth, they should only get to feed after the nb. so then the milk usually comes in faster, maybe engorgement, (supply = demand remember)but the elder child can relieve this by feeding for you.
theres lots of info on the aba website - article - breastfeeding pregnancy and beyond. i haven't tandem fed (yet) but thats what i read there
hth's.