lessm is very similar unto lessc except for the fact that it accepts multiple input files.
lessm essentially takes the input files passed to it and creates a file with @import statements for each file. It creates these @import's in the order that the files were passed to it. Since order does indeed matter in LESS land, this is important and order is guaranteed.
Right now usage is as follows:
lessm foo.less bar.less bing.less -o out.css
The -o argument is optional. If specified, it will write the output to that file, otherwise it will output to stdout.