R5 lunar photography with stacked EF extenders

PhotosFlight wrote:

A little help please. I have the 1.4 MII and 2x MIII. I'm missing something because it feals like I would be forcing it were I were to stack them.

The EF 1.4x II is not as sharp (optically) as and EF 1.4x III Extender because it's an older design that was produced for pre-digital lenses.  Adding one to an EF 2x Mk III Extender would result in (A) softening of the image from the EF 1.4x III Extender and (B) a further reduction of contrast and loss of image resolving due to the longer focal length of the 2x Extender being coupled with it.  We do get more actual magnification from stacking two extenders together but there's a cost.  The trade-off is in the amount of light lost and the optical consequences of putting too many glass elements between the subject and the sensor.

Edit. Ok, I went back and see it now. I guess I need an adapter but really I should first upgrade the 1.4 MII.

Canon wanted to discourage people from double-stacking Extenders.  Canon expressly DO NOT recommend double stacking Extenders because doing so results in poor contrast which in turn affects exposure and focus.  But of course, people kept doing it. 
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So Canon redesigned the Mk III Extenders (which are designed for Digital cameras and modern lenses) to prevent it.  Part of the optical redesign to improve image quality resulted in a slight increase in the front element and this required a rubber buffer to prevent people from damaging the rear optics of lenses that were not designed to accommodate an extender.  Many lens buyers don't realize that extenders will only fit on a select number of lenses so they try to cram them onto anything they want, often severely damaging their expensive lens in the process.
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Older Mk II Extender can be stacked together but the optical quality is markedly inferior due to an older optical and mechanical design, older optical coatings and a slower microprocessor.  These were pre-digital Extenders that were released to service lenses made between the 1980s and the 1990s.  For more recent Mk III Extenders, an EF12 (tele-tube) needs to be placed between the two Mk III Extenders to allow them to safely connect.  Note that Canon still do not recommend double-stacking Extenders.
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* Tip:  Most owners of Mk III Extenders don't realize that the lens caps for the front end have been redesigned ever so slightly to accommodate the slightly longer protrusion of the front optical element.  Getting the front end lens cap mixed up with your other lens caps will result in not being able to safely seat the front lens cap on a Mk III Extender if you grab the wrong one and attempt to remove an extender from a lens whilst in the field.