You are right. They are not in the same namespace, but they are in the same assembly so you should not have any problems to use it like this:
table.AssertTable().RowRange(ArtOfTest.Common.NumberRangeCompareType.OutsideRange, 1, 2);
If you are having difficulty to make this to work, please provide more information on the issue you're experiencing. It would be best if you can provide us with a sample test for repro.
the Telerik team