Is there such a thing as a PVC pipe reducer end cap fitting, which has a center hole and an offset hole?
The purpose of the center hole is to insert a small diameter straight PVC pipe through the center of the cap, so that the smaller pipe is projected inside the larger pipe.
(This smaller pipe is used to inject a gas or fluid into the larger pipe, using additional PVC fittings inside the larger pipe.)
The purpose of the offset hole is to provide a drain for the larger pipe, aligned with the edge of the larger pipe so it can be drained completely through the offset hole, into another PVC pipe.
So for example the large diameter PVC pipe would be 4" schedule 40, and this end cap would have a 1.5" center pipe hole without an inner stop-lip, thereby allowing a pipe to project through the cap, and a second 1.5" offset hole for a PVC drain pipe.
I know there are often special industry terms for certain objects, and I do not know what this might be called. Possibly such a thing might be available as a hot tub PVC plumbing component.
EDIT: Some mockup drawings in SketchUp
The outer PVC pipe diameter should be 4 inches, but I do not care specifically what the smaller pipe sizes are. I will work with whatever I can find.