This depends upon your phones. My Galaxy S9 has the ability in Settings/Connections/Data Usage to set a hard limit (data shuts off) at a specified threshold. You'd have to do this for each phone individually. It also has a warning option.
In my case, I'm more concerned about the shared data usage across our two phones. You can see this information in the Mobile app, but I wanted some graphical. I installed a third-party app (My Data Manager) that includes a bar graph widget showing the combined usage during the plan cycle, and shows individual device usage within the app. It allows me to allocate data to each device and generates warnings, but I think the warnings are just for the combined usage rather than individual usage. It does require installation on each device then linking into a combined account. There are plenty of other apps out there with similar funcitonality, but I wanted the widget on my home screen.
What I really want is something like the Verizon data widget, that shows a speedomet-style graph of shared usage in a glance and is based on the provider's data, and I've submitted such a request to customer service, but so far it isn't available on Xfinity.
