class CompanyDal(ISession session) //nhibernate session
{
    Save(ICompany company)
    {
        session.Save(company)
    }
}

public class TenantConnectionProvider : DriverConnectionProvider
{
    private IDictionary<string, string> _tenantConnectionStrings;

    public override void Configure(IDictionary<string, string> settings)
    {
        // Load connection strings from config file
        _tenantConnectionStrings = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
        foreach (ConnectionStringSettings connectionStringSetting in ConfigurationManager.ConnectionStrings)
        {
            _tenantConnectionStrings.Add(connectionStringSetting.Name, connectionStringSetting.ConnectionString);
        }

        base.Configure(settings);
    }

    public override IDbConnection GetConnection()
    {
        var connectionString = GetConnectionString(); //not shown, mine is for Web API
        var connection = new SqlConnection(connectionString);
        connection.Open();
        return connection;
    }
}

// connection string has to be set even though custom provider is used
var config = Fluently.Configure()
    .Database(MsSqlConfiguration.MsSql2008.ConnectionString("custom").Provider<TenantConnectionProvider>)
    .Mappings(m =>
    {
        m.FluentMappings.AddFromAssemblyOf<MyClass>();
    });

.Register<ISession>(factory1.OpenSession()).Named("f1");
.Register<ISession>(factory2.OpenSession()).Named("f2");

.Register<IRepository>().OnActivating(e => {
var db = HttpContext.Current.Request.Url....//some code to find the db
var dep = e.Context.Resolve<ISession>("f1");
    e.Instance.SetTheDependency(dep);})