var dstuff = from user in selecteddb.users
             where user.UserEmail == userEmail
             select user.UserID;

public static class ConnectionTools
{
    // all params are optional
    public static void ChangeDatabase(
        this DbContext source,
        string initialCatalog = "",
        string dataSource = "",
        string userId = "",
        string password = "",
        bool integratedSecuity = true,
        string configConnectionStringName = "")
        /* this would be used if the
        *  connectionString name varied from
        *  the base EF class name */
    {
        try
        {
            // use the const name if it's not null, otherwise
            // using the convention of connection string = EF contextname
            // grab the type name and we're done
            var configNameEf = string.IsNullOrEmpty(configConnectionStringName)
                ? source.GetType().Name
                : configConnectionStringName;

            // add a reference to System.Configuration
            var entityCnxStringBuilder = new EntityConnectionStringBuilder
                (System.Configuration.ConfigurationManager
                    .ConnectionStrings[configNameEf].ConnectionString);

            // init the sqlbuilder with the full EF connectionstring cargo
            var sqlCnxStringBuilder = new SqlConnectionStringBuilder
                (entityCnxStringBuilder.ProviderConnectionString);

            // only populate parameters with values if added
            if (!string.IsNullOrEmpty(initialCatalog))
                sqlCnxStringBuilder.InitialCatalog = initialCatalog;
            if (!string.IsNullOrEmpty(dataSource))
                sqlCnxStringBuilder.DataSource = dataSource;
            if (!string.IsNullOrEmpty(userId))
                sqlCnxStringBuilder.UserID = userId;
            if (!string.IsNullOrEmpty(password))
                sqlCnxStringBuilder.Password = password;

            // set the integrated security status
            sqlCnxStringBuilder.IntegratedSecurity = integratedSecuity;

            // now flip the properties that were changed
            source.Database.Connection.ConnectionString
                = sqlCnxStringBuilder.ConnectionString;
        }
        catch (Exception ex)
        {
            // set log item if required
        }
    }
}

// assumes a connectionString name in .config of ADBEntities
var selectedDb = new ADBEntities();
// so only reference the changed properties
// using the object parameters by name
selectedDb.ChangeDatabase
    (
        initialCatalog: "name-of-bdb-initialcatalog",
        userId: "jackthelad",
        password: "nosecrets",
        dataSource: @".sqlexpress" // could be ip address 100.23.45.67 etc
    );

using (var context = new MyDbContext("Server=localhost;Database=dbA;..."))
{
    return context.Users.Where(u => u.Email == "someemail@google.ca").Single();
}

var connections = ConfigurationManager.ConnectionStrings;

foreach (ConnectionStringSettings connection in connections)
    //display connection.Name

ConnectionStringSettings selected_connection = connections[1];

using (var selecteddb = new ADBEntities(selected_connection.ConnectionString))
{
    var dstuff = from user in selecteddb.users
         where user.UserEmail == userEmail
         select user.UserID;
}

var defaultString = _myContext.Database.GetDbConnection().ConnectionString;
_myContext.Database.GetDbConnection().ConnectionString ="new connection" or _myContext.Database.GetDbConnection().ChangeDatabase()
//your query..
_myContext.Database.GetDbConnection().ConnectionString = defaultString;