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As stated there is no such rule.

However, there is some contention as to whether or not you are locked in combat by the first unit to successfully make it into base contact (you can search for the threads that go into more detail).

 Ond Angel wrote:
So if I'm charged by 2 units, Unit A charges then B. If I want to overwatch, I have to do it at unit A? If I refuse to and he doesn't make the distance, I then have to fire at unit B IF I choose to overwatch?
Am I getting that right?

More or less.

Unit A declares a Charge, you decide whether or not to Overwatch.
If Unit A fails it charge, and you did not declare Overwatch, when Unit B declares a Charge, you can Overwatch.

If Unit A successfully charges, and you did not Overwatch, you may or may not (depending on your reading of the rules) fire Overwatch at Unit B.

If it wasn't obvious, I'm trying not to start the argument back up, which is why I'm expressing the broad generalization of both sides.