Our best guess is that you generate s single item set from your content file and then share it for the both combos. While this is possible, it will not behave correctly, as those items have two owners (the two comboboxes), which is not best practice - the items inside the second combobox will have the first as their owner which will lead to incorrect theming, fault hit-testing and/or other problems in the behavior.
If your case is of other kind, please write us back and provide us with a sample project (you'll need to open a new support ticket) so we are able to investigate the problem deeper. Otherwise, if you need further assistance on your case, don't hesitate to write us - we'll be glad to help you.
the Telerik team