Thanks to LeftOfCenter202, whose idea in 101 Ways to Drive a sub Crazy Group inspired mine. You can find his method in the Group or on his post BDSM Christmas.
You can spank two submissives at the same time and make them count how many spanks (s)he received (positive number) and how many spanks the other sub received (negative number).
For example, sub A got spanked 36 times, sub B got spanked 32 times.
A should count +36 and -32.
B should count +32 and -36.
This will be mentally challenging to keep separate if numbers of spanks are close for the subs, i.e, 36 and 32.
It is a different kind of difficult if the numbers differ widely, i.e., 32 and 14.
sub A will wonder why (s)he is getting less and what sub B has done to deserve double the punishment while sub B wonder why (s)he is getting more, and if sub A is a better sub than (s)he is for getting less. In short, it becomes a mental play for the two.
With this double counting system, it is irrelevant if the Dom keeps count. The subs should write down both + and - numbers they counted after the session. If they do not match, then both earned themselves some more punishment. If they do match, the subs might feel good about each other and more willing to play together again under the same Dom. Win/Win.
After-care required for both submissives.