You can only have 1 main and 1 support job at any time and you only get the abilities from those 2 jobs you do not get your 2 hour ability for your support job.
You cannot get a support job until you hit level 18 and do the quest to learn how which is pretty hard to do, or not really hard but requires a group and patience for items to drop.
Im level 20 Warrior/9 Monk atm.
You can switch your jobs at anytime tho, your support job can only be half the level of your main so if I was 20 Warrior and 15 Monk the best I could do would be 20 Warrior/10 Monk, you dont lose the levels, if I became 22 War I would be 22 War/11 Monk.