Even with a speedy mammy-daddy achieving an even snare-drum roll can still elude many drummers.
Well, truly it is because speed has nothing to do with the production of an even roll, even though when practicing that often will seem to be the main objective.
Actually it is really the control of the bounce technique that has the most effect and speed is not really necessary to produce an even long roll.
In fact you will probably find that the faster your wrists have to move when making a long roll, the more likely it is that it will sound uneven.
It may be better to think of it rather than mammy-daddy but stroke-bounce, stroke-bounce by making a downstroke where the stick does one bounce for each downstroke.