The double beat use of the metronome proposes that earlier composers used slower tempi by means of using the metronome differently then our current use.
In the double blue beat use of the metronome there are two parts of one beat: the back and forth of the pendulum oscillation is one event like the inhalation/exhalation of a breath, the systole/diastole of the hearth beat, or two halves of a day.
When the metronome number indicates the minim note the ticks are not beating the minim note but the crotchet note. When the metronome number indicates the crotchet note the ticks are not beating the crotchet note but the quaver note.
The metronome numbers are consistent through many of the original editions of works by composers of the time, but taken in modern use of the metronome many of these tempi seem too fast, even in slow movements.
Many M.M. verge of been unplayable, and some are physically unplayable.
Playing the music in double beat does not result in halving the tempo, based on the fact that modern recordings don’t follow the speeds indicated by the composer, and are generally slower.
If the double beat theory is applied then all the original metronome marks make sense.