Is it necessary that it fills the one first? Or could it be filling both and shut off each one individually as they fill? Basically with a T on the output line and 2 float switches.
If its absolutely necessary to fill one before the other, do you have head on the fish room tank to fill the kitchen one with gravity? If you did you could fill the fish room one and drain into the kitchen one, closing that off when its full with a float. This would mean that the kitchen one gets filled from the fish room one, which might not be desirable (depending on how clean you keep it).