1- You can export to PDF as spread page then place in indesign on A3 page size.
2- Create a new document A3 size, then select and copy all the items on the two spread pages and paste on the new document.
The "correct" way to do a project like this is to set up as facing A4 pages, and to lay out in consecutive page order as a reader would see the pages (reader's spreads), with a single pages for first and last pages. When it is time to print you can either use an impostion script, or File > Print Booklet... to "impose" the document into the printer's spreads for output onto A5.
From Print Booklet you can either print direct to a duplexing printer, or on PC you can print to PDF. For mac you would need to print to Postscript file and distill manually, which is a pain.
A third option is to simply export to PDF in single pages, then use Acrobat's booklet function to print.