[Kirk Y]

Any modification you make to one style will have to be transferred over manually to the other. The only exception to this rule is when you have 1 parent style and multiple child styles of this parent style (ie. Parent Style is stock vBulletin style, Child Style 1 is a green variation, and Child Style 2 is a red variation). In this instance any modifications you make to the Parent Style will apply to its Child Styles as well.

The Parent/Child setups are possible only when the differences between the Parent and Child styles are CSS only -- so don't try setting your new styles as child styles of the Default Template.

Back to the point, you'll just have to apply any changes you decide to make to one template, to all of them - there's really no reliable shortcut.