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We've added option "keep fronts and backs together" that controls this. Check it off to get the "old" behavior.
With the latest version here you can both edit trim box and shift crop marks and trim line.
Done by renaming the "Booklet" option in the latest version:
Please check our latest beta version of Imposition Wizard: - the feature is done there (you can also drag and drop PDF files into the application).
You can edit trim box with the latest IW: - bleeds are not editable, as we automatically cut them in case of intersection.
The latest IW lets you both correct bleeds and shift crop marks: