If you have used the Entity Framework then you know the EDMX is cool. You also know it can become HUGE and almost unmanageable.
When it gets large, it is tempting to create a second EDMX or third - even one for each Schema in your database (just as an example).
Such a seperation would help with organization of your EDMX, but it could seperate the context of entities in the same namespace.
Moreover, seperate EDMX files can create a situation where a JOIN operation across EDMX files results in excessive, redundant database communication.
But, the fact remains, the larger the EDMX, the more difficult it is to use. The more difficult it is to ensure it is correct. The easier it is to break.
Do you break your EDMX files apart? Do you have a rule of thumb for when to it?