How do you stop iPhone from changing iPad Wi-Fi network?

[aces99]

Hi, I am having a problem I need some help with. I have a Wi-Fi modem in my living room that covers one side of the house and I have a Wi-Fi extender in the family room that covers the other side of the one because the signal is to weak if you are in the family room and hooked to the main modem. The problem is if we are using the iPad in the living room and hooked to the "Main" modem and I take my iPhone in the family room and hooked up to the "Extender" the iPad also switches and hooked up to the "Extender". How do you stop the iPhone from changing iPad Wi-Fi network as well when you move the iPhone and hook up to a different Wi-Fi network.