How to control mutilpe validators on one property?

We really recommend using INotifyDataErrorInfo for validation.  It's async and supports rich error info.

Here's a blog post series I put together for RIA Services validation - much of this applies to Silverlight even if you're not using RIA Services.

http://jeffhandley.com/tags/RiaServicesValidation/default.aspx

Also, you should look at the Validator class in DataAnnotations:

http://jeffhandley.com/archive/2009/10/16/validator.aspx

-Jeff