I might have some input again :-)
I got robert of cops911 to make and call it a castor troy holster, and then he ''gave'' the design to specops back in the day, for the original castor troy for two zeus pistols. It definitely was a spanking machine when you run.
Also pball pistols are huge. Two of them on your back with any bit of angle, which makes guns(the size of pball guns) draw and reholster comfortable, will have their grips stick out past your hips. A big problem with holstering pball pistols is that they will stick out too far in most positions.
You could mount them lower so they can still be drawn and holster at more of a vertical angle, but then they would be even more of a spanker.
Guns the size of ours mounted up high in the small of your back, like Troy has his 1911's, would need a double jointed shoulder to get at.
So what do I do now?
I have an older skunkworks holster (pre retention strap, only the trigger guard for retention) on a belt holder in the middle of my back, left handed draw, at a comfortable angle for a gun that long. To draw: my left thumb rests on the belt holder and pushes, popping the trigger guard out of its lock. To reholster I just find the holster with my barrel tip and slam it home. Kydex is awesome for reholstering one handed where you cant see the holster.
I really like being able to quickly and confidently empty my hands to do other stuff. I hardly ever need a quick draw, and even if I do its not that bad. But being able to help kids with their masks or reloading is nice.
So I pretty much always have this left handed rear holster on me, whether its for a tac8 as a secondary to a pump or if I have two tac8s on me.
The primary tac8 goes into a right handed shoulder holster (left arm pit). You have to find an acceptable angle and height that's comfortable for you. That may mean putting it more on your hip or just letting it stick out and be careful when you snap shoot with that side.
There is one other draw that I like for such long guns, ill have to find the video for it to really make sense but I will try to describe it.
Gun holstered on your hip with the barrel parallel to the ground, under your draw hand.
Your hand rests on the holster and draws the long gun forward, grip butt out and away.
This puts your index finger around the trigger guard. You kinda roll ''drop'' the gun, so that the trigger guard rolls onto the index finger.
Then you twirl it the rest of the way into your grip.
I know this sounds really complex, bits its only three hand motions and works for really long pistols or with a silencer tip. In fact this is the only draw that was comfortable with my joker gun (drv with a 14" barrel)