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My opinion for what its worth is the concept of 2 separate scopes on the same mount with a tandem bar type mounting is engineering-wise never really going to work.
To get tight stars the guiding has to be what the sensor sees and the more removed it is from that the more variable and error can be added in from the many sources of flex in a system that the TPoint model shows.
I presume you are using Protrack corrections as well? That may help.
Personally I believe you would be better off using 2 mounts rather than trying to make a tandem system work. Are there examples of others using tandem systems like this that work well? Usually its a smaller scope riding on top of the larger scope. Like a guide scope. Even then there are likely to be small differences.
I think you've got a great result already from a setup I would not have thought would have worked at all. The TEC180 is a large scope and quite long. Its angular momentum will be quite a bit more than the TEC140 perhaps that could be part of it. Are you sure your setup is balanced at different angles? It would be tricky to get 2 scopes to be balanced at all imaging angles which may be part of why its been erratic in certain parts of the sky. The weight load is not even perhaps.
When I had a TE180 a Tak NJP could not quite handle it but the PMX could with MMOAG guiding. Guide scope guiding worked sometimes but was less certain and often eggy stars. A PME11 should handle it easily but then a TEC140 which is also quite a large scope as well?
Greg.
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