Your sub job won't get any exp, only the main job gets it. So if you want to lvl your sub job you have to make it your main job.

Remember that you don't loose any lvls on your sub job, you just don't get its abilities/spells/stats.. If you have summoner at lvl 30 and thief at lvl 30 and you decide to be a thief(main)/summoner(sub) then your summoner lvl will be 15(30/2=15). When you get lvl 32 thief then your subjob (summoner) will be at lvl 16. And when you get at lvl 70 with your main job (thief) your sub job (summoner) will stay at lvl 30, you will have to make it your main job in order to lvl summoner again.