The only way I know to combine 2 beams of the same wavelength is to use a PBS.
I'm running 2 BluRay's and 2 Long Open Can Reds. To combine more than 2 is not impossible, but there are some sacrifices made. The beam shape is oval shaped due to the beams riding side by side and the beam diameter is hugh.
In the end it's not really practical to combine more than two beams IMO.