How to add results of two select commands in same query

Yes. It is possible :D

SELECT  SUM(totalHours) totalHours
FROM
        ( 
            select sum(hours) totalHours from resource
            UNION ALL
            select sum(hours) totalHours from projects-time
        ) s

As a sidenote, the tablename projects-time must be delimited to avoid syntax error. Delimiter symbols vary on RDBMS you are using.