Thanks for your answer. But not clear yet, sorry.

Two questions:

1.. general case: When we know about an oncoming power loss (line works, f.i.), is there a way we can regulary shutdown both nodes and bring them up after powerback as a cluster? In which order must we shutdown, and in which order must we repower? Or have we to stop the cluster, to stop all raplication tasks before, and to re-instantiate all single facts after repowering?

2.. in our case of a arbitrary power loss, we found all volumes on both sides as destinations. That's right. We then changed the appropriate ones to sources and started all replication tasks sucessfully. All good until then. After some time, when all replication tasks were in sync, we started the cluster, and this failed, more exactly, the process of starting never ended up, so we had to reboot the systems to finish this deadlock. After reboot, the state was again with destinations on both sides. We had to do the same again, with no success. Then we called Open-e support, and they succeeded after hours and two more failed tries with some magic via remote support. Is there any way to solve this without Open-e remote support? Remember that we successfully reinstantiated all replications, and all was in sync. All things looked same like when we created the cluster. But here, we couldn't start it.
The Open-e technician didn't understand this too.