it seems that you need imposition software ..
I assume that you need to arrange a bunch of pages in a certain sequence.
if you intend to do a booklet (let's say of 8 pages) so that last page (8) is facing the first page (1) and the page (7) is facing the page (2) ... and so on.
and you will bind the booklet from its center in our case it will be the centerline of an A3 folding line.
this is a case if you wish to bind a booklet from its centerfold of the A3 paper.
here you may need to combine your A4 paper into on PDF file and use one of the imposition software available there are many, some of them are stand-alone like KODAK PREPS Imposition and some of them are just a plugin into Acrobat like
Quite Imposing Plus
I really like the plugin as it is easy to use.
but if you want just to arrange the photos in a sequence that page (1) is facing page (2) and the page (3) is facing page (4) and so on. you don't need to use the imposition software, even though it is easy the imposition software. bit actually there so many ways that you may achieve the same result, I will name two
first, you can print the A4's on an A3 paper using Acrobat PDF and the output could be to the printer directly or to a PDF file
in the above screenshot you may first choose the printer you want and select to print on "Multiple" or "booklet" and make sure to choose 2 pages per page.
if you want the output to be PDF so in the printer choose "Adobe PDF" and then enter into the properties of the Acrobat PDF printer and choose A3 as output paper in the "Adobe PDF page size" section
The second way is to use Adobe bridge and it will open photoshop contact sheet script. (of course you may use it directly from photoshop by going to File > Automate > Contactsheet II )
open acrobat bridge and browse to your project folder
choose two A4 Pages you want to merge into one and go to Tools > Photoshop > contact sheet this option will make a contact sheet with the selected pages .. make sure you select only two pages
and make sure to enter the A3 size in cm (29.7*42) and two columns and one row and the hereunder screenshot
Tell you the truth there are so many ways merging two A4 into A3 paper .. so many ways that I can not collect in one answer. some use scripts and some use software like Photoshop, CorelDraw, Illustrator, and similar to them some use data merge like InDesign, Illustrator, Photoshop, and professional imposition software, so many ways even I can merge two pages into one using excel and/or PowerPoint. it is a very common task and so many software can do so many even I cant imagine.