Thank you for your answers,
the mentioned IcyDock thing won't provide me with additional SATA Ports, so this would be no help.
But I found those two things:
http://www.jj-computer.com/artinfo.php?artnr=A%20%20%201436
http://www.raidsonic.de/en/products/ssd.php?we_objectID=8206
I bought the first one, only to find out, that only the first drive is recognized when connected via SATA. So this dock needs a port multiplier capable SATA controller as stated in the descripion of the raidsonic dock. I think there's no (cheap) solution for putting two SSDs in one drive bay / one SATA port.
As for the Seagate Archive drive, they are formatted with 64K Clusters as I read your suggestion in another forum post. The SSD keeps a write speed of about 90MB/s whereas the HDDs begin with 100 MB/s and fall down to about 35 MB/s in the middle. The Seagates keep their speed for about 8 GB before they slow down.
I hoped to use one SSD as landing zone because of Christopher's statement here: http://community.covecube.com/index.php?/topic/1178-how-files-are-duplicated/&do=findComment&comment=7867
I understood it the way that realtime duplication writes on the landig zone and simultaneously on an second drive with the result that the slower drive determines the write speed. If realtime duplication is not enabled, the all data are only written to the landing zone and duplicated later. Is there any configuration possible to achieve this behaviour or do I misunderstand the functionality of the duplication mechanism?
Michael