Yes. If you level your black mage to 14, then go your mog house and change your main job to Redmage, and level it to 10, and change back to your black mage, it'll work like this:
When you get your subjob quest completed and change Back to "redmage" to level it up a little, you'll be a level 10rdm / 5 blm (regardless of your current max level of black mage, as long as it's 5 or more).
If you, however, complete your subjob quest after seperately raising Red Mage to 10 and Black mage to 18, and just go "add" Redmage as a subjob, you will be an 18 blm / 9 rdm.
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Elahna
24 Blackmage
24 Whitemage
10 Thief
Garuda Server