If you often make many more than the 50 post maximum, and there is an open spot, I don't see why DS would not be willing to give you a second spot, so that you would be eligible for payment from 100 posts/week, provided the additional 50 posts you make each week have been consistently better than other applicants who were applying for open spots.
This wouldn't be a good idea and definitely isn't fair to others. I'm sure there are lots of people who can, would or do make more than their paid amount, but if this became a thing there would be a hell of a lot more people trying to go for multiple spots. It's best just to keep it at one per person and if someone makes more than their allotted 50 post maximum then that's not only just a bonus for Chipmixer but also proves that that person isn't posting just to get paid. Either way, I would be very surprised if Darkstar started doing something like this. He might as well just up the maximum post count to 100 or something to make it fairer but 50 post a week is a pretty nice amount and works out to 10 a day 5 days a week. Any more and people would probably start forcing themselves to make more posts which isn't good.
For clarity, while I do think it should be okay for someone to enroll alts in a signature campaign, I also believe this should either be disclosed (at least privately to the campaign manager), or for there to not be a rule prohibiting this. Enrolling two accounts owned by one person is not something that should be done if the campaign manager is not aware, or if there is a rule prohibiting this.
Most people aren't going to disclose their alts unless it's already public knowledge, but they don't do that because most campaigns strictly prohibit multiple accounts so that's why people keep it on the hush hush. I very much doubt things are going to change regarding that or how people treat 'cheaters' of campaigns. It's best to just not get greedy and keep it to one account per campaign or certainly don't get caught but there's likely always going to be consequences for those that do.