I have two text boxes:
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This is to represent orbital information around a body, the apoapsis (A) is the point when you're at the furthest distance from the body, and your periapsis (P) is point of closest distance. What this means is that your A cannot be less than your P (though they can be equal, maybe never in the real world though).
So what it comes down to is when a user is entering this information, they should not be able to enter a value for A which is less than P or vice versa. If they attempt to, is it better for the change to be blocked, with some feedback, or intelligently switch the values of the two text boxes, and move focus to the appropriate text box, so the user can continue what they're doing?