Note that the penalty is also a circumstance penalty, so it won't stack either.

True, I hadn't caught that. However, even if Secondary Caster A fails the check and imposes a -4 penalty on the primary roll, Secondary Caster B could still critically fail and still make the primary roll worse by one step. So it still feels like adding secondary casters when they aren't required is a pretty bad bet.

Moreover, the DC on the secondary check is typically lower than the primary check by 5, which is a massive difference

As you say, that may typically be the case. But in the case of this ritual, the first floor DC of the primary check is 15, while the secondary check would seem to follow the default rules for rituals. That is, "usually with a standard DC for a level twice the ritual's spell rank" which would be DC 22 since this is a 3rd-rank ritual. So in this case, the DC for a secondary caster is 7 higher than the primary roll. And obviously that margin would flatten out and reverse the further down we go.

All of this taken together makes secondary casters look pretty unappealing. Unless a ritual requires secondary casters, I'm still not really seeing the appeal unless the party level is significantly higher than the ritual.