New day, new tests... 
I have three SSD's: two 840 EVO (A and B), and one EVO 850 (C). Two of them (B and C) are not able to boot at all, neither alone nor in any combinations.
Test cases I've tried
- A in slot 0, B in slot 1 --> failure
- B in slot 0, A in slot 1 --> failure
- A in slot 0 alone --> success
- A in slot 1 alone --> success
- B in slot 0 alone --> failure
- B in slot 1 alone --> failure
- A in slot 0 , C in slot 1 --> failure
- C in slot 0 alone --> failure
- C in slot 1 alone --> failure
As soon as I started using two traditional HDD's everything went smoothly. SSD's are not the friend of MBLD.
Checking the logs, I could find the difference: disks with DRM support will definitely fail to boot. No matter which slot this disk is in, and no matter if alone or in pair.
"supports DRM functions and may not be fully accessable."