In the manual it is said all 4 ways are possible although I see them used as in the photo all the time.
By design the Twin6 Be’s are rather intended for a landcape orientation, though in some situations they could be positioned
vertically. As seen in the earlier rear panel description, the Twin6 Be has a switch allowing to choose which drive unit is to
reproduce the mid frequencies (see below) (fig. E). Consequently it will always be possible, as desirable, to configure the speakers
in a “mirror” (ie symetrical) way (fig. F). As a general rule having the “midrange” driver on the inside of the cabinet results in
better imaging.