cp -nrv A/* B/which simply places all files in (subdirectories of) A/ in B/ and vice versa, skipping files that already exist. If the transfer is interrupted and restarted however, it will not resume the copying of a file that had been partially copied earlier but otherwise this is a very simple and easy way to get the two-way archiving job done!
cp -nrv B/* A/
Unison File Synchronizer has served me well over many years, helping to keep my work and personal life up-to-date on several computers in at least three different locations. Lately, however, I have been trying to keep a very large collections of large files in sync on two different external hard drives and Unison is just too slow for this because it is so thorough. If you are essentially just adding files to a collection, the much easier way to keep directories A and B synchronized is
By / Deur Petrus