How to change error message of one customvalidator in another customvalidator function

I used two textboxes in gridview (One for cheque number and other for cheque date). I used two custom validator for each textbox. 
My conditions for validation is:
1> Both textboxes can be empty.
2> If user enter value in one text box then it is mandatory to enter value in other text box.
As per above conditions how can I perform validation so that I get mandatory message if one text box is empty.