First idea, get a splined hub that fits over inner shaft, machine it with a thread on outside of hub, machine a thread on the inside and loctite the boss in, problem is the outer PTO shaft is very thin walled and integrity of shaft may be compromised.
Second idea, Machine a splined hub and weld onto inside of outer shaft, problem is It ruins the outside PTO shaft if anyone wanted to put back to original, also there is an oil seal that I need to install which would not be possible as welded splined hub would be in the way.
Third idea, get an outer hub and machine it and weld it to the clutch plate splined hub.
Fourth idea, weld the two shafts together as what the majority of people do, but compromises the integrity of the inner input shaft.