I thought this all sounded familiar...
I have a
front-mounted SM60 II
filter (so this may be different to your situation if you have the scope) and have been looking at double stacking this the "traditional way". Some searching and corresponding with Agena Astro about a year ago resulted in the following:
1. You need the SM60 double stacking etalon with RichView tuning (the one that's not suitable for single stack use (Meade has confirmed this)
2. You need adaptor plate AP186 (Hans is correct and Meade has confirmed this also)
3. At the time, Meade
would have been able to supply the etalon
but not the adaptor plate. And no-one knew when it would be available (a common problem resellers have with Meade - they are notoriously unreliable with items they don't have in stock at a given moment). Note that the adaptor plate is not rocket science. I've had a similar adaptor made by the guy I bought my FS60 from in Europe. This was needed to attach the SM60 to the FS60.
4. Is factory matching required? Not necessarily, but remember you are mating two elements both of which are subject to normal variation in production. Individually, that may not be a problem, but by combining them you may be compounding 2 initially unproblematic errors to make one that's beyond the tuning system's ability to compensate. I imagine this would happen if both elements were to deviate from the norm in the "same direction" if that makes sense. But I'm not an engineer. Suffice to say it is a
slightly risky approach.
This may or may not be of help, but I thought I'd share it here.
Ken & others, I have another idea for a DS solution, which I'd likle to run past you some time & ask your opinion.
$0.02 - YMMV