Hi Russell,
Sorry for the delayed response.
Your requirement can be achieved by displaying AutoComplete list for each entries in the TextBoxExt separated by comma.
This can be done in AutoCompleteItemSelected and AutoCompleteCustomize event handlers.
Please refer the below sample that demonstrates the same and let me know if you have any further queries.
http://websamples.syncfusion.com/samples/Tools.Windows/F68436/main.htmRegards,
Jaya