Depends on orchestration.
I'm with Wave on this one. In a rock context I'm never playing doubles on the toms. It just drops the volume and attack too much.
Even sounding and spaced notes is basically my entire goal typically when I'm working on doubles. Good finger-work (or wrist if slow enough) is required to get volume out of the second stroke.
Definitely true, but rock drummers use singles on toms. That was my point. Even in a fill that incorporates double strokes, those doubles tend to be on a snare in a rock context.
In a gospel, funk or fusion context, sure. But in rock, you will sound like a fusion drummer if you start playing double strokes on your tom fills.
I do it all the time, and trust me, I've never been accused of being a fusion drummer. Especially for stuff on the floor tom, it's usually more efficient to play two on my right hand than bring both over. As long as you really snap the second stroke and don't just let it flop down with gravity like I see some guys do, it sounds just fine.
The only way that the second stoke of a double can be louder than the first is if you are going slowly enough to make two separate motions, and snap the fingers on the second stroke.
This does not occur at high speeds. At high speeds it becomes a single, compound motion employing rebound.
You've got your thinking a little bit incorrect here... Never should you not be in control of both notes in a double. Regardless of speed. The big difference between fast and slow doubles, at least for my playing, is that slower is usually mostly wrist... Faster is when I start using my fingers more to snap the stick back down after the rebound. But you gotta really snap that sucker to get the right volume and sound.
I flog it here all the time, but rudiment exercises such as the "long roll" from slow to fast really bring home the point of making sure both notes in a double sound even throughout the speed range. It takes a lot of practice, and the concept isn't really apparent to most newer drummers until they start to learn from an explanation or teacher.
And again, it is often a choice based on things like sound, dynamics, and physical limitations of only having two arms that decide if a set of two notes will be with one hand or two. Typically it is not speed that makes the decision for me.
For me it depends on the dynamic desired, the sound desired, the surface I'm playing on and the speed. That's a lot of variables!
That said, having all of your options open with some pretty dialed in hands offers more choices and makes choosing easier.