To make a long story short — don’t do it, it won’t work. 🙂
And now the long version of the whole story. The following applies to Debian 4.0 aka “Etch,” but it’s not specific to Debian, since the principles of the whole procedure is the same on all Linux distributions.
For the first time I wanted to use a partitionable RAID1 array, because I wanted to spare myself the trouble of creating multiple partitions on both drives which would then be combined to individual RAID1 arrays. Instead I would have to create a single partitionable RAID1 array and partition it afterwards.