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Differential flexure is always going to be a problem no matter how rigidly you mount your two scopes together. The flexure between the two OTA's is fixed by making them "as one'' with suitable "bonding". However the flexure caused by small movements in individual mirrors, focusers etc is much harder to control.
The only satisfactory method is to guide from the same optical path as your camera. This can be achieved with an off axis guider. However this limits the selection of suitable guide stars.
At the ASNSW photo group meeting this month we were shown a way of using a combined method of using one guider to control the other to correct errors. This allows extremely long exposures without any errors caused by flexure. Software to help this is available. If you live in Sydney I can recommend this group. Meetings times and dates are on the ASNSW web site.
Barry
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