Accordion display as attachment to primary accordion display won't display properly

Problem/Motivation

I have created two similar accordion view displays of fields both grouped by a taxonomy term id, but with a different relationship for each, and sharing the same exposed filters. No contextual filters. Ajax is enabled. Both accordion displays work separately as blocks on the same page, but I need them to be triggered by the same set of exposed filters so the users don't have to fill their request twice.

Steps to reproduce

When I set the second display as an attachment to the first, and check "Inherit the exposed filters": the main block appears just fine, but for the secondary view, only the accordion header (rendering the taxonomy term name) is displayed (oddly with 2 triangle icons) and none of the content fields.

I can't figure why and if I've done something wrong!

N.B. I also tried to place both displays as blocks on the same page (or one as page and the other as block) with the exposed filters as a block and only the first exposed filters block displayed on top of the page to command both displays (the exposed filters have the same identifier), but it won't work either...

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes