My suspicion is that you never changed the port assignments in the configurations. Server B was probably left running; your attempts to start Server A failed because it couldn't get the port already held by B.
So, first get both running by editing the buildout.cfg for one of them to specify a different http-address for the zope instance.
Once both are running, you may combine them so that they appear at different paths in the same web host by using rewrite rules in your reverse proxy (probably apache or nginx).
If you want to combine them into the same Zope database by exporting one site from it's original zope server and importing into the target server. They will still be different Plone instances, though in the same database. If you want to combine them into the same Plone instance, then export content rather than the whole site, and import into the target Plone site.
Export/import is done through the Zope Management Interface. The two Plone configurations must match exactly for this to work. Expect to have to reindex the Plone content after any export/import.