Thanks, I've got a Skywatcher guidescope Mount, so I'll start with that. I discovered when I was piggybacking my two scopes that, because of rings etc not being exact, both scopes ended up pointing fairly close to the same point! I wasn't happy with how the eq6 was handling so I've shelved things pending my new mount.
I discovered the finder shoe with the finderguider was loose when I was dismantling everything, which probably accounted for my guiding issues! But I also had problems with the top scope not being usable too near the zenith because of the design of my dome so I was going to try a tandem setup like yours next time.