The official webpage says

Assembling DjVu Images into Multipage Documents

The batch compressors distributed as part of the DjVuText and DjVuLayered packages can directly produce multipage DjVu file when fed with multiple input files. The files produced are smaller than if the pages are compressed separately because the compressor can extract and share redundant information accross multiple pages. Individually compressed DjVu pages can be assembled into multipage documents using the free package DjVuMulti. To assemble a bunch of DjVu images into a single BUNDLED document simply type:

djvubundle page1.djvu page2.djvu.... pageN.djvu document.djvu To

assemble a bunch of DjVu images into an INDIRECT document, type: djvujoin page1.djvu page2.djvu.... pageN.djvu documentdir/index.djvu where documentdir must be an existing directory where all the individual page files will be copied. To disassemble a BUNDLED document into an INDIRECT one, simply say:

djvujoin document.djvu documentdir/indexfile.djvu 

To convert a multipage document from one of the old 2.0 multipage formats, do djvureindex olddocument newdocument The programs djvujoin, and djvubundle supersede the 2.0 programs djvuindex and djvumerge.

I couldn't find djvujoin and djvubundle for Ubuntu. djvulibre doesn't have them either. Do I miss something? Thanks.