These are some examples showing how to get a poster as you want.
For these examples we use two input pages:
These are intentionally uncommon formats so the effects of running pdfposter will be more demonstrative.
Portrait images are higher than wide.
Example 1:
pdfposter -p 2x1a4 testpage-tall.pdf out.pdf
This are two a4 pages put together at the long side: Two portrait pages wide and one portrait page high.
Example 2:
pdfposter -p 1x2a4 testpage-tall.pdf out.pdf
This are two a4 pages put together at the small side: One portrait page wide and two portrait pages high.
Landscape images are wider than hight.
Example 1:
pdfposter -p 2x1a4 testpage-wide.pdf out.pdf
This are two a4 pages put together at the long side: Two portrait pages wide and one portrait page high.
Example 2:
pdfposter -p 1x2a4 testpage-wide.pdf out.pdf
This are two a4 pages put together at the small side: One portrait page wide and two portrait pages high.