Spenceroo,
What I have always done, and I'm sure all will agree with my practices, is that any plain jane inner paper sleeves with no writing on them, or the ones that may show just other releases by the manufacturer (EMI-Capitol, RCA-etc) I usually dump as quickly as possible.
Those that have Pictures, and/or Lyrics, and information about the particular release, definitely do keep them, and in general, I have usually found ample room to place these inside the LP Jacket with the LP, and new Inner Sleeve without too much crowding, or difficulty getting the LP in, and out.
If you should personally find a release, in which it is just too tight a fit to fit both Inner Sleeves, then yes, place the saved original inner Sleeve outside the Jacket, and place at the rear of the Jacket within the Outer Sleeve for protection, and safe keeping for future reference.
Hope this helps. Mark