RadFilter's idea is slightly different from that of building this kind of custom filter scenarios. A single field editor is expected to produce a single filter expression with one of the generic filter functions. When you want to build more complex expressions you add more field editors, each adding up a part of the final result.
If you need to offer this kind of UI, I would advise you to manually build a filter expression and use the combo with checkboxes separately, without integrating it with RadFilter.
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