You are correct, but still it wont work like that. Something was missing and after analasys I saw that the Start_Date field wasn't NULL but just empty. So this is what I changed to make it work:
,Datediff(SECOND,
convert(datetime, ISNULL( NULLIF([START_DATE], '' ) , END_DATE) + ' ' + ISNULL( NULLIF([START_TIME], '' ) , END_TIME)),
convert(datetime, [END_DATE] + ' ' + [END_TIME])) AS M_Time_Worked
A big thank's to all for helping me and the responses where I learned from. Kind regards, Ambertje