I didn't know you can do that. This is not supported, you should use EXISTS instead: pass null to the field paramter and specify a filter:
bucket.PredicateExpression.Add(new FieldCompareSetPredicate(
null, null, OtherFields.Item, null,
SetOperator.In, (SomeFields.Item == OtherFields.Item & SomeFields.ItemDate == OtherFields.ItemDate)));
which will generate:
WHERE EXISTS
(SELECT OtherTable.Item
WHERE SomeTable.Item = OtherTable.Item AND SomeTable.ItemDate = OtherTable.ItemDate)