BibTeX files can be sorted by specifying the command line option -s. The given files are sorted according to the reference key. Several files can be given at once in which case BibTool will sort and merge those files.

bibtool -s file1.bib file2.bib

With the command line option the files are sorted in reverse ASCII order.

bibtool -S file1.bib file2.bib

If you want to sort the BibTeX files according to the authors then the following invocation should do the trick:

bibtool -- sort.format="%N(author)" -s file1.bib file2.bib

This means that the sorting order is determined by the (normalized) author field.

BibTool Manual Introduction Using BibTool --- Some Instructive Examples Sorting and Merging