Cant separate 2-spline PTO shaft in order to cut it

[RickB]

How does one secure a short cable like that cable inside a PTO shaft?

Obviously it was pretty well attached as the OP mentions applying a fair amount of force when attempting to pull the shaft apart.

I have seen cables looped around the rollpins that secure the tubular shafting to the yokes.

 

[LouNY]

I have seen cables looped around the rollpins that secure the tubular shafting to the yokes.

Yep, the most common way