Which of your 2 rabbits is this 6 week old? Or is there now a 3rd rabbit in the mix?
You just adopted a "2 year old mix" whos been from home to home, so theres no way shes 6 weeks old.
If this is the 1st rabbit your friend gave you, that you got the end of september being the 6 week old one, that would put the rabbit at 3 weeks old when it was given to you, in which case i sure hope a rabbit that young was taken from its mom, but then youve posted pictures of a black rabbit which Im assuming is Simba? in which case Simba is not a Dutch.
So thinking neither of your rabbits are actually 6 weeks old, you can safely just go get revolution or advantage for BOTH of them. The dose for revolution is 18mg/kg.
Be sure to change all bedding, vaccuum your floors, wash everything thats fabric, and freeze that which you can't wash. DO NOT use flea collars, flea bombs, flea shamopps or sprays, topical treatments from the pet store, wipes etc...
As per the water, how much is alot? What are you feeding them, how are their poops?