I don't quite understand what you meant by "Will it level up faster", but you must level you're White Mage seperately from your Red Mage, SubJobs do not gain EXP whilst you have them set as you're SubJob.

SubJobs can be no higher than half of your main Job. If your White Mage is higher than half of you're Red Mage when you set it as a SubJob it will be reduced to a level that is exactly half of your main Job. If your White Mage is lower the half the level of your main, the level of your White Mage Remains the same while set as a SubJob.

It's a very good idea to keep up to date with the leveling of your SubJob because if you don't then you are what is known as a "Gimp", and from my experience some party members don't take too lightly to a fellow gimped member, So keep your SubJob up to date!