With doubles you don't need a manifold, but you may want one.
The manifold makes gas management and fills easier. If you're diving anything other than air, an open manifold assures you that you have the same gas in both tanks. Partial pressure blending in a set of doubles with an open manifold is just like doing it with one tank. Toping is simple, but with two independent tanks that have different remaining pressures, you may have to empty both tanks if you're topping with a different gas or such. There are endless scenarios, but the bottom line with gas is, doubles with an isolator is just like dealing with a single tank, and independent doubles is like dealing with two.
When diving doubles with a manifold, the tanks will breath down evenly, so there is no futzing around with switching back and forth between two tanks. You can isolate them if you want and have independent doubles, so you have a choice. But with independent doubles they are always independent.
Think about air shares with independent doubles verses ones with a manifold. Which reg will you hand off? Can you use a standard doubles configuration with one long hose and one bungeed back-up? Sure, you should always have enough gas in each independent bottle to get your buddy to the surface, but many times during the dive the amount of gas in each bottle will be different, making air sharing gas management more difficult, verses just handing off a primary reg which you know has the two of you sharing all the available gas.
It's why you can hit a boat or beach with a crowd of divers with doubles and be hard pressed to find independents. Sure, it possible, it's divable, and in some travel situations it may be the only option. Buy if you have a choice, is it preferable?
Not for me.
