I have a CustomValidator that is validating a telephone number for several different telephone numbering schemes. The client-side javascript looks like this:
Just remove all instances of Text and ErrorMessage in the custom validator tag definitions then use
$(#id_of_custom_validation_control).text("custom error message");
Then with this, you can use one custom validator control and handle multiple error conditions
E.g
if(error1){
$(#id_of_custom_validation_control).text("custom error message1");
}else $(#id_of_custom_validation_control).text("custom error message2");
Hope this helps