A PBS cube will take an unpolarized beam in one side and will split it into two polarized beams; one vertical and one horizontal. If you take two polarized beams (which the blu-rays and reds typically are) and go the other way, you can shine them into two faces of the cube and they'll come out of a third face overlapping/combined. In order for it to work you have to rotate each laser to the correct angle so that the polarization is correct. So one will be horizontal and one vertical. The combined beam will have, of course, both horizontally and vertically polarized components, so if you try to pass this through a second PBS, only one of them will get through. That's why you can't use multiple PBS cubes to combine more than 2 beams.