I appreciate two-timing piano students who practice their scales with acutely sensitive ears. They are made keenly aware of what it takes to repeat a faulty step-wise sequence that's been thrown out of rhythmic alignment along a 4-octave route. (Auditory memory is a vital ingredient through repetitions that require retrieval of a basic underlying pulse.)
In a journey from 8ths to 16ths to 32nds, many pupils will underestimate the end game tempo, losing technical control in the final spill. To avoid a pile-up in the speed zone, they will put on the breaks, losing their initial framing beat. Ironically, a good proportion of two-timers who find themselves in such a jam will "think" they've doubled-up in the 32nds range, only to discover by a teacher's real-time demonstration, that 16ths to 32nds were out of synch. (A metronome can be just has helpful in clarifying rhythmic disparities.)
Ways to deal with rhythmic disorientation
I prompt students to back up by "half" from what they can realistically manage in 32nds. After a few retrograde repetitions in this practicing mode, they can revisit 8ths and then move forward again in doubled sequence. In most cases, a pupil comes to grips with what he can safely control at the peak scale build-up, knowing that the underlying pulse will increase in developmental stages.
A recent lesson sample illustrated rhythmic disproportion and remedy. (It's excerpted at the juncture where a student zoned in on 16ths to 32nds in a D-sharp minor Harmonic form scale) A brief second segment focused on a "rolling into" effort in a more fast-paced staccato-rendered scale in Melodic form. It was a confidence-building effort that represented a "rite of passage" for this pupil who realized that she could, in fact, play brisker 32nd notes without faltering. Breathing, pacing, mindfulness, and lack of PANIC all kick into controlled, peak playings.
Original Content: Two-timing scale practice