Ok, so a 'lot' of strange things are happening in Imposition for me with this application. I developed this on version 6.2.8 and I see you are on 6.1.6. The attached workflow should import and work.
HOWEVER, it will ONLY work for the size 307x80mm. To get it to work at all, had to set some specific dimensions. The 'Auto' doesnt work for this.
What you are doing is typically called Asymetric Imposition. Core supports this with the Output Management license. However, for the record, I tested it there also and it is NOT working how I think it should work. Trying to see if I can get the other size you sent to work. If I can find a consistent issue then we might be able to submit it as a bug and see if they will fix it. Just not sure right now...seeing so many different things.
For now give it a try and let me know. I renamed the 'main' imposition preset so it wont overwrite yours.
307x80mm_impose_final.pdf - is as it should be.
307x80mm_impose.pdf - this what is Core doing, there is one piece missing and it is shifted
Under the area that you type in text, you should see an item to 'Choose File'. Select that and browse to the zip. Then select Open (bad terminology). This will attach it to the message then you select Post.
Can you please export a workflow and provide a sample file - we should be able to help with this. But need to see exactly how you are doing it. The files should be .zip to upload easily. ideally show a picture or image showing how the 2 portions of the page should be laid out.