create a composite schedule, and assign that to the job when you create it.
for example:
begin
dbms_scheduler.create_schedule('MY_SCHED1', repeat_interval =>
'FREQ=DAILY;BYDAY=TUE,WED,THU,FRI;BYHOUR=6,10,14,18');
END;
/
begin
dbms_scheduler.create_schedule('MY_SCHED2', repeat_interval =>
'FREQ=DAILY;BYDAY=MON;BYHOUR=8,10,14,18');
END;
/
begin
dbms_scheduler.create_schedule('MAIN', repeat_interval =>
'MY_SCHED1, MY_SCHED2');
END;
/
now you'd just assign the "MAIN" named schedule to the job instead of the schedule string.