SE5 gun arrangement

[Honza]

Why they didn't use a twin gun?
Could the pilot fire both at the same time? What was the reason to keep it like that in 1918? Does it have any advantage apart from firing upwards?
Just realized how weird this arrangement is.

[Killer Moth]

Yes, pilots could fire them both at the same time. My understanding is that the dual, synchronized fixed guns weren't well-trusted, because of their jam-propensity, and being able to shoot the ventral side of your opponent with the Lewis Gun was appealing.

[Spoonfrog]

According to Wikipedia/!\ the Nieuport 17 had the same arrangement...

[flash]

Why they didn't use a twin gun?
Could the pilot fire both at the same time? What was the reason to keep it like that in 1918? Does it have any advantage apart from firing upwards?
Just realized how weird this arrangement is.

No stranger or weird than the Nieuport series of planes with two guns.
Albert Ball is well known to have liked the Nieuport and to use the Lewis gun firing upward, as did some other pilots. The AFB-1, in which he had an influence in the design, would have had the same set up if it had come to fruition.
Another advantage was that you could load incendiaries & explosive rounds when needed in the Lewis with it's open bolt operation rather than having them in the Vickers where it's believed they were at risk of cooking off in a hot breech of the closed bolt operation of the Maxim, or engine compartment. The Fokker D.VII had some issues with that.
There was also a question over the reliability of the hydraulic synchro gear for the Vickers initially, so it was a comfort to have a second gun system independent of that.
Both guns could be operated from triggers on the stick, the Lewis connected by a Bowden cable as well as by it's trigger when pulled down to fire upwards.