ciao mario,
if you have only one layer:
- create your flyer as A5.
- when you're finished, copy all elements, create an A4 landscape document, paste two times the content, place it correctly and generate the pdf...
otherwise:
- create your flyer as A5
- export it to pdf
- create an A4 landscape document with 0cm margins
- page > rown and columns > manage guides and create 1 column guide
- insert two image frames
- load the pdf in each frame
- export to pdf by using the experimental PDF embedding feature
nermanders' hint to use multivalent is also very good...