I don't think you save anything by booking one ticket rather than two separate tickets, so I would simply book two separate tickets.

I think the only way you can get a London-Duesseldorf ticket is by calling Rail Europe. You simply change trains at Brussels onto the Thalys service.

Indeed, if you allow plenty of time for the connection at Brussels then you might even save some money by booking the ticket from www.thalys.com (Thalys is the high speed train service on the Paris-Brussels-Cologne route) and saying you want to collect the ticket from the station in Brussels.

Rail Europe and Thalys can also mail you the tickets if you have a UK address and the credit card is registered to that address.