Jim, finish driers are often formulated in combination because individual drier chemicals are classified as primary (top) driers, or through driers.
Using a primary or top drier alone could result in a finish that feels hard and dry on the surface, but is still soft just below the surface. This could result in damage, or create problems if you thought a stock was dry and hard enough for checkering. Using only a through drier chemical, or using too much drier additive can cause a brittle finish.
The calculations of how much of each drier chemical to use for a given batch of finish are somewhat critical and they can get fairly complex when several drier chemicals are used. That's a reason that it may not make a lot of sense to mix your own homebrew, unless you are working from a known good formula, when the manufacturers have gone through the trouble to correctly work this all out for us.
