[AccessD] Replicated .mdb and SQL Server

I currently have an Access application, split into a front end linked to 2 separate back end .mdbs.  Backend "A" contains the majority of the data tables for the application; Backend "B" is a replicable database that is also used as the backend database for another Access application that is used by salesmen on their laptops.

The client is considering migrating the data for this application to SQL Server.  Any ideas on how to handle this?  The front end contains some queries that currently join tables from both Backend "A" AND Backend "B".

Thanks,
Barb Ryan