Batch merging two TIFF files into one

I have a lot of portrait B5-size TIFF files that were scanned from paper documents and I plan to scan more.
I want to merge every two of these files into landscape B4-size TIFF files.
I want to do this by a batch job.

What is the best way?
Are there any command-line tools to do this job?

I do not have Adobe package software. Instead I installed MacGimp.
Although I have some experience with shellscript and Applescript,
I'm new to Script-Fu.

I made a GUI script for the application Preview that reads a multi-page TIFF file and saves as a PDF file with 2-UP layout. This method has two flaws:
(1) Preview aborts for large files.
(2) Resulting PDF file is more than five times as large as original TIFF file.
Namely, 40-page, g4-compressed, 400KB TIFF file results in 2.5 MB PDF file.
Maybe, the Preview does not make inheritance of TIFF compression.

So I decided to make multi-page TIFF files of 2-UP layout.

Thank you for reading.


iMac G4 Mac OS X (10.3.9)