I think that you need to suck this one up. If they need to pee, they need to pee. Perhaps they need to be engaged more so they are preoccupied with something that really holds their attention. Another option is to tell them that there are set times that they can have a bathroom break. I appreciate that you have seen an increase in your water bill but seriously I am just not convinced that it is that dramatic from just a few extra flushes per day. I imagine that you are frustrated more than anything but seriously to restrict a child from going to the bathroom seems like a huge waste of energy. I have way to much to be worrying about before being concerned with how many times a child goes to the bathroom. In fact, I'd be more concerned about their health and the why's of their frequent bathroom visits. If these were my children I would also insist that the toilet be flushed in between children as it just isn't hygienic especially with young children, even girls, who manage to get pee everywhere. You are in charge of what goes on in your house so set some ground rules on when bathroom breaks are and maybe even reduce their intake of fluids to set times too. If they haven't used the bathroom much at school during the day then it makes sense that the frequency in which they go would increase after school. Its not as simple as just getting to your place and having 1 big pee it for sure could be a case of a couple of bathroom visits before the bladder is emptied. If it isn't to do with this and they just go to the bathroom together all the time just because, then remember that it is your house, your rules. If they go together, just don't let them. It would be interesting to know how old the pair are too...also are they siblings?