Thanks useful information: 
The design of Canon's extenders ( including 3rd party for Canon ) means that you can't directly couple two extenders together because the protruding glass element physically doesn't fit. The way to get around this is to use a 12mm extension tube between the two extenders
i shall add, you could couple a 2x with a 1.4x , but not a 1.4 x with a 1.4x , or a 2x with a 2x if that makes sense
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