A redirect creates a new request which is always a GET, therefore you can only pass parameters in the URL. Obviously you do not want to expose information in the URL because it will be visible.
One solution would be to use a database visible to both servers, create a single-use random ID and store the information you want to pass with that ID.
A JMS (
Java Message Service) server visible to both would also work and has the advantage of automatically removing the message.
Bill