In grade 3 music theory I learned about compound time and how in 12/8 for example you have 4 dotted crotchet (quarter note) beats per bar. Each beat them gets 3 quavers (8th notes). But what does it mean when a beat is notated as two dotted quavers? Does this mean that in that instance, the dotted crotchet is temporarily halved giving us 2 quavers per beat and thereby suspending compound time temporarily? I say temporarily because it is supposed that a beat should be divided into 3 for compound time so by dividing it in 2 it is like suspending compound time temporarily.
In 12/8 the beat is normally divided into three quavers. But, just as in a 'simple' time (2/4, 3/4, 4/4...) we might want a triplet, in a 'compound' time (6/8, 9/8...) we might want a duplet.
I wouldn't say that compound time is 'suspended'. One cross-rhythm doesn't change the meter any more than one chromatic chord changes the key. And other instruments may be continuing the compound groove at the same time.
Traditionally, this would be written with a 2-bracket. But a dotted quaver is half a dotted crotchet, and there is a growing trend to write a pair of dotted quavers instead.
I prefer the duplet as giving a clearer indication that 'normal' grouping is being overridden.
Gould agrees with me. Doubtless some people here will not!
12/16 would be compound quadruple time with dotted quaver beats. 12/8 would be compound quadruple time with dotted crotchet beats and the rarely used 12/4 would be the compound quadruple time signature with dotted minim beats.
All of them have four beats, all four beats having dots, the only difference being what consist of a beat, dotted quaver vs dotted crotchet vs dotted minim.