The above post basically explains what happens.
Since the sj can only be at max 1/2 of the lvl of your main job, by lvling whm, u'll probably make Bard your man job and WHM your subjob.
You'll have the same access to all the spells and skills your WHM has at that lvl. So if your bard is lvl 22, your whm will be lvl 11, meaning you can use all the WHM spells and skills up to lvl 11 (eg Cure2).
If your bard is lvl30 and your WHM is lvl 15, that allows you to use divine seal.
the other abilities you are not allowed to use of your SJ class is your 2 hr - in this case benediction.
I know I played a RDM to 30, went Bard but SJ whm, so now I'm lvling my WHM too, heh. It means I have a lot of RDM spells that I'm not using - but they're still theref or if I ever go back to using RDM or another job that overlaps spells with RDM.
75BRD, 75BLM, 75WAR, 72RNG, 65THF, 62NIN, 42WHM, 41RDM
Kayile On Fairy