I have lost my patience...
I have to build up my system as soon as possible so I decided to make the RAID1 on a fresh install which is pretty straight forward. I'm not sure if the partman(standard partitioning tool invoked during debian setup) can create GPT, in my case it didn't, therefore I used gdisk or similar to create the GPT table on every disk which I want to use in the array (fdisk cannot handle the GPT). If you proceed building up your RAID1 based on MBR instead of GPT you won't be able to install GRUB2, this is really annoying since installing GRUB2 is almost the last step in the installation process. To be able to boot from any device of your array you just have to install GRUB2 on each of it using this command
I know this is not an option for people who need to make RAID1 on a running system but in my case it was easier to build up the whole system on a fresh install then getting RAID1 on a running system.
have a nice day!