If you have complex logic to implement you could
write a custom validator inheriting ComapreValidator and write all the validation logic you want in the validation method.
public class V1 : CompareValidator
{
protected override bool EvaluateIsValid()
{
this.ErrorMessage = "message";
}
protected override bool ControlPropertiesValid()
{
return true;
}
}
to mantain simple the code, uncheke "Enable client validation" or override the property setting false by default, so that javascript validation in turned off.
If you enable client validation you have also to implement javascript logic...
Hope this help.