chickens with diarrhoea

Last week 1st chicken went down with it she seemed withdrawn but the last two days she has looked ok in the morning but tired in the evening she has been eating and drinking,now this morning a second chicken was lookin withdrawn and tonight when I got home shhe was in the nest box when I picked her up pooo just poured out of her.
I have isolated both of them what else can I do

thanks in advance

DES