A while ago, there was a discussion here about enforcing readmore tags in long posts. It was basically shot down under that node as difficult to implement (though a few other reasons were mentioned).
The neat thing about discussions around here is that they can be shot down completely one week, and then brought up by somebody else in the next week and ++'ed to one of the Best Nodes.
I have seen people that maintain the code here comment that it wouldn't be too hard to write something to auto-add readmore's. The harder task is agreeing how it should work.
I ++ you for presenting your idea, even though I don't think I like it. It relies on the end user to know how to do the right thing with both of those text boxes, just like the present system relies on the end user to know how to use readmore's. Better to find a progmatic solution.