I have desired condition now, but with W8.1
But the problem I had before was not caused by the Windows version, so one could argue whether the thread is really "solved" because I still don't know if it works with W10. It didn't have anything to do with multiboot too.
My problem was, that the Windows Installer (since Vista) allways creates the Windows bootloader on the drive which is set first in the boot order of the BIOS, even if this is not the drive on which Windows is installed. In my case this was the HDD and not the SSD on which I installed Windows. Because the the HDD is on port0 and the SSD on port1, the HDD would alway be set first in the boot order by default.
So the Windows partitions were on the SSD, but the bootloader was created on a automatically created partition on the HDD, which I didn't notice first.
Seen this, I first tried to trick the Windows installer by formating the HDD with gparted in a linux format which Windows doesn't know, so the Windows installer sees it as "full" and uses the SSD for the Windows bootloader. But this just led to an error message during Windows installation, that said that it couldn't create a partition or so.
I had to change the boot order in the BIOS and in order to make the SSD appear in the boot order, I had to set it first in the "Hard Drive BBS Priorities".
Then, with the bootloader on the right drive and one of the partitions encrypted, I encountered a last problem which was about multiboot:
In the Windows bootloader, the encryptet Windows was set as the default one, because it was the second one I installed. So after skipping the Truecrypt authentication with "esc", the Bootloader tried to "repair" the encrypted partition. It didn't do any harm, but I had to wait everytime till it gave up the "repair"process and let me boot the unencryptet Windows.
So I had to change the default operation system entry, following these instructions:
Oh God, this took so much time, including the whole thing with changing the BIOS and the Windows setup from UEFI to MBR mode. But I'm happy now!
One last flaw resides:
If I want to boot the encrypted Windows and booted the unencrypted one before, the bootloader menu appears and after selection of the encrypted Windows it always restarts the Computer after selecting the encrypted Windows and I have to type my Truecrypt password for a second time. Not till then the encrypted Windows will start directly.
Does anyone know why this is and whether one could change this?