If I understand correctly, it sounds like what you're trying to do is more of a UI thing than DevForce. You might want to take a look at INotifyDataErrorInfo interface and see what event handler you can customize to achieve what you want to do. 

Also if you're binding to a DataGrid, each cell's error is handled by the following: 

"The DataGrid supports cell-level property validation and row-level object validation. If a validation exception is encountered in the property setter, the cell editing control displays its error state. The DataGridCell.IsValid, DataGridRow.IsValid, and DataGrid.IsValid properties are all set to false. The DataGrid will not exit cell editing mode until the validation error is resolved."

          For example, let us say we have FromDate and ToDate properties on an entity. Is it possible to trigger validations when either FromDate or ToDate is changed (which of course, is possible), but notify only ToDate in either case so that only ToDate is shown as in error. Is there any trigger setting which can accomplish this? 

          Currently (DevForce 6.0.6), both properties are shown with error marking (red box around - INotifyDataErrorInfo) on the silverlight client, and the ValidationSummary (Silverlight control) reports the same error twice as both properties are in error.