I’m basing this answer on the way other spells behave when being split or twinned. References to a single target apply to both.
This is for Hold Person, for example.
The target must succeed on a Wisdom saving throw or be paralyzed for the duration. At the end of each of its turns, the target can make another Wisdom saving throw. On a success, the spell ends on the target.
With the spell twinned, now target refers to both creatures. I see no reason for the action to continue the spell to not apply to both as well.
Furthermore, here’s a tweet by Mike Mearles in response to a similar question.
One spell instance, two targets.
That tells me, the Split spell continues to act as close as it did before but with the added second target. You are not concentrating two spells, and you don’t need to spend an action on each creature, one action works for both.
See Witch bolt for a similar case:
On a hit, the target takes 1d12 lightning damage, and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically.
If twinned, your action lets you damage both the new targets. The wording is a bit different but wording of spells isn’t always 100% consistent for what I’ve seen. I think it’s very likely Crown of Madness is intended to work the same way.
As for the other questions:
- If just one action is sufficient to maintain control of both targets, can the caster elect to end control of one target but not the other?
No, you can’t. If we treat it any other spell with multiple targets, say for example an upcasted Hold Person, by RAW, you can drop concetration at will, ending the whole spell, but there’s nothing that let’s you end only part of the spell.
- If the spell ends on a single target (either by choice or through lack of a required second action), does the spell automatically end on the second target? (because of the ‘or the spell ends’ clause)
No, it doesen’t. Again, see how spells with multiple targets work, when each target has a separate condition to break the spell, like a saving throw or moving away, only those that have fulfilled this condition are free from the spell.
- How might the spell be reworded to demonstrate the RAW effects on two targets?
Replacing each instance of “target” with “each target” might work, “any target” may be even clearer.
Note that this doesn’t make the spell particularly clearer for a normal reader without a multicast feature. Which may likely be the reason why the spells aren’t written that waty to begin with.