From what Deris said, I would anticipate this being more of a permanent thing. As long as they can pull it off money-wise (having two singers on the payroll can get expensive), I see no reason why they couldn't. Deris' comment about not having to perform the high pitched Kiske songs is hysterical. I think Andi should stick to his own era, honestly. I never cared for him singing Kiske-era tunes anyway.
So if it works out that the tour is successful, and they all get along great, and they attempt writing something together, and that all goes great, then if the money issues get sorted, I'd be pretty happy if this was permanent.