You can set secondary job as a sub command. However without it set you can still use abilites from the board.
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anoymous
You can only use the Active Skills under the same job as the Sub Command you set (except the first two basic skills).
Not sure about the Passive Skills though.
Good way to check this is look at Mont's skill list in battle.
Unlock "Body Slam" and/or "Treatment" (Lord Skills) in the board, then Equip "Paladin" Sub Command.
In battle, you will not see "Body Slam" nor "Treatment" in the Skills list.