Planewave

These "double images" are normal and are caused by the tilt of the two filters. They cannot be avoided and actually are the reason why the filter works so well. We have explained this in more detail on the Important Notes for our (also double-stacked) K-Line Filter. Please check out the following PDF:
https://www.baader-planetarium.com/en/downloads/dl/file/id/8/product/1399/important_notes_on_the_stacked_k_line_filter.pdf

As for your second question: A 2" double-stacked Solar Continuum (or K-Line) Filter unfortunately is not possible, because it would require a much larger filter cell than our normal cells.

Thank you for using our Baader Planetarium which we now produce in the 53rd year.
Unfortunately we only have a few cosmetically imperfect solar caps left from the plastic molded version that we used for long years.
Today we are using a glass-molded cap, because it is more durable. But this would require to modify the lamp holder of your planetarium - and it would be quite expensive to send the middle part of your planetarium (without the star globe) to us in Germany for doing the modification. So if you were to accept the plastic cap with a few dimples, we could send it to our US-distributor for Baader astronomical products (www.AlpineAstro.com) so that he could send it to you at an economical outlay of altogether US $ 25,-

Please let us know if this would be acceptible.

The MaxBright II has two front-connection options:
1. A female T-2 thread to adapt onto most common photographic equipment
2. A Zeiss ring-dovetail as it has been established by Carl Zeiss for more than 50 years.

We will soon publish an extensive manual that shows the multitude of accessories that we recommend to combine with the MB II in order to adapt this onto most every telescope with the shortest possible optical length.

We cannot equip the MB II with the star diagonal that you certainly would need with your refractor in order to have a convenient way of looking through the telescope, because all users of Newtonian telescopes could not make use of this star diagonal. Accordingly it will be a matter of the customer or the dealer to help finding the appropriate adapters or star diagonals for the telescope in question and most of all, to enable this adaptation without wasting precious backfocus. This is the reason why we have developed a multitude of T-2 prism star diagonals with T-2 threads on both sides. Because you could choose for your refractor whether you want to use a 2" nosepiece to put it in front of the star diagonal or whether you opt for a 1.25" nosepiece.

So please have a bit more patience until we publish the MB II manual and check our website for star diagonals. Because furthermore we are preparing an extensive document to explain all the differences and benefits of our various star diagonals in exhausted detail, this will also be published within the next month.

You need this product where your asking from (2" DSLR Filter Holder #2408166) as well as our Hyperion DT-Ring SP54/M49 #2958049
https://www.baader-planetarium.com/de/hyperion-dt-ring-sp54m49.html

The sharpness performance is definitely better. Only the attenuation through the glass is comparable. In the FFC there are two elements of pure fluorite crystal. This is outstandingly transparent at 350 nm.
But there are two more elements from regular molten glass also in this lens collection. Therefore the pure amount of "normal" glass is about the same.
Nevertheless the transmission at 350 nm should be 90%.