If you use waveguide to connect multiple dishes this would be a very complicated setup, and the satellite signals would combine to cancel each other out, that is if there was enough signal received at the LNB to begin with. The more plumbing between the feedhorn and LNB the more loss.
I wouldn't suggest doing this unless you really like to experiment.
The Orthomode feedhorn would be a better option, but another option is utilizing a dual polarity LNB on the original feedhorn, just make sure you get one with the same mounting ring as the feed horn.(Inverto Pro black LNB, they come in single and dual output).