Forcing a cell to change value

That works if I want to notify someone in C1 with the "error message" but is there a way that one cell will autmatically =0 if the other other is >0 and vise versa?

The best solution is to prevent entry of a number if the other cell already has a value greater than zero. You can use Data validation, custom otion with a formula like so:

For cell A1: =B1<1
For cell B1: =A1<1

Customize the error message as required when setting up the Data Validation.

Note: the above formula do not prevent negative numbers.