How can I reuse a Common Table Expression

If you do not require them in 2 different queries, you can try

;with query as (
  Select Row_Number() over (Order By UserID ASC) as TableRowNum,
         FirstName,
         LastName
  From   Users
),
totalCount AS (
    SELECT COUNT(1) Total FROM query
)
Select  query.*,
        Total
from    query, totalCount 
where   TableRowNum 
between 1 and 25 
Order By TableRowNum ASC

If you do require 2 different queries, rather use a table var

DECLARE @User TABLE(
        TableRowNum INT,
        FirstName VARCHAR(50),
        LastName VARCHAR(50)
)
;with query as (
  Select Row_Number() over (Order By UserID ASC) as TableRowNum,
         FirstName,
         LastName
  From   Users
)
INSERT INTO @User
SELECT  TableRowNum,
        FirstName,
        LastName
FROM    query

SELECT  *
FROM    @User
where   TableRowNum 
between 1 and 25 
Order By TableRowNum ASC

SELECT COUNT(1) FROM @User