As far as I know, View button code cannot be accessed. As a workaround, you can do the following:
Add a textbox in your report and let the text be something like "Please enter either cmb1 or cmb2" .
Now from the Database check if both Values are entered. If yes, just return the columns in your select statement as NULLs or specify a value e.g.
EMP_NO = 9999
.In your report if the returned value for EMP_NO is 9999 by checking
FIRST(fields!xx.value ) = 9999
you can determine what did the user enter.Add an condition to the textbox visibility checking if the value is 9999 or not, do the same for other items in the report to hide them and just show the textbox.
This can be used for other validations as well.