Possible to combine several bz2 archives into one?

You can simply concatenate many bz2 files into single bz2 file, like that:

$ cat file1.bz2 file2.bz2 file3.bz2 >resulting_file.bz2

bzip2 and other utilities like lbzip2 will be able to decompress the resulting file as expected.

If you're willing to burn a few days of CPU, here's one solution with the magical pipe facility of modern UNIX(R) operating systems:

bzip2 -dc file*.bz2 | bzip2 >resulting_file.bz2

... actually, grab lbzip2 version 2.0, and do the same, except with lbzip2, on a multicore:

lbzip2 -dc file*.bz2 | lbzip2 >resulting_file.bz2

You should flip the question around - you should not try to decompress and then recompress the files, simply make a tar archive of all the separate files - tar is ideal as a container for the separate files.

tar cf tarofbzfiles.tar *.bz2