you mean similar issue https://issues.jboss.org/browse/RF-9567 appears? - there was a problem that ajax requests for data loading invoked validators. If so please create new issue for calendar.
Or the problem that you just can't get value from second calendar to validate both? That standard JSF trick and could be solved for example like there http://blogs.nuxeo.com/dev/2009/03/cross-validation-with-jsf.html