Set up as facing A4 landscape pages. In Print Booklet you would need to set the page size to A2 and set the positioning to centered in the printer setup dialog. In thory I suppose it should be possible to print two-up, but the only way I can think of to make that happen would be to set up a new A3 portrait facing pages file and place the A4 into it twice (use Scott Zanelli's script from InDesignSecrets » Blog Archive » Zanelli Releases MultiPageImporter for Importing both PDF and INDD Files), then use print bookle on that file.
The bigger question in my mind is do you have a printer that can use A2 sheets? Id yo are sending this out, le the printer do the imposition from a your A4 file exported normally as sequential single pages as PDF.