private CustomerDataContext dc = new CustomerDataContext();
private SysDataContext ds = new SysDataContext();
public IEnumerable<MiddleSellerOrg> SelectSellerOrgBySellerID(string SellerID)
        {
var q = from a in dc.Cust_SellerOrgs
                    join b in ds.Sys_Organizations on a.OrgID equals b.OrgID
                    where a.SellerID == SellerID
                    select new MiddleSellerOrg { 
                    ..........
                    };
return q.ToList();
}

private CustomerDataContext dc = new CustomerDataContext();
private SysDataContext ds = new SysDataContext();
public IEnumerable<MiddleSellerOrg> SelectSellerOrgBySellerID(string SellerID)
        {
var q = from a in dc.Cust_SellerOrgs.ToList()
                    join b in ds.Sys_Organizations.ToList()
                    on a.OrgID equals b.OrgID
                    where a.SellerID == SellerID
                    select new MiddleSellerOrg { 
                    ..........
                    };
return q.ToList();
}