Combine CTE “WITH” and a “WITH XMLNAMESPACES….” in SQL Server

Use a comma instead of the second WITH, e.g.

WITH XMLNAMESPACES('http://schemas.myself.com/SomeSchema' as ns)
,CTEQuery AS
(
SELECT (list of fields)
    FROM dbo.MyTable
    WHERE (conditions)
)
SELECT * FROM CTEQuery

The same if you want multiple CTE expressions. You only need to specify WITH once, and then all other WITH blocks just use a comma instead of the keyword.