Now why didn't I think of putting it directly into the tool holder? Duh. I can turn a single bushing and slit it. Wasn't sure that would work any better. Yes the bolts are present and they have tapered flats to draw against the bushings. And I did position the bushings so the bolts bore directly on the slit, at 90° to the slit and at 180° to the slit. No changes noted. And I stopped short of putting a cheater on the wrench for tightening and I put all I could muster into the tightening. I do have another boring bar holder with bolts that squeeze two sides of a slit together for holding. I guess a double bushing is just not an effective way to hold something. Thanks for the input.
DanK