Durst negative carriers are a modular system which the end user can "customize", as you've already find out.
When you're using both metal masks, the "Sivoma" goes on top and the "Sixma" is the lower one. But remember that they're supposed to be mounted with writings OUTSIDE the negative (flat side OUT). You'll see that it makes sense as there's a retaining clip which holds them on the edge, and it wouldn't work if the writings were toward the negative.
The glass/metal combination is a Durst classic. The suffix "Sivo" suggests you that the glass shoud replace the UPPER mask. You can then leave the Sixma mask below. That's what Durst suggests, but if you want you can as well experiment a bit by swapping the metal mask and the glass and see if you like better to have the glass in lower position.