You can combine almost any color of lasers if you have the right optics. I have a lot of specialty pieces that I use in RGB projectors as well. Polorisation is a physics attribute of the diode's bar size as well as the dynamic structure of the wavelength that is propagated in the crystal doubling characteristics. Most all lesers will need to be rotated upon it's axis to be combined / split with a dichroic or a PBS cube.
Hey there is an ultra cheap way too! First, if you notice a beam is a good deal smaller near the laser, and even the best will spread out as they travel. If you have a mirror with a small spot of the reflective stuff removed, arrange so that one beam reflects (and is aligned with the second, very near the one that passes through the "hole" in the mirror, as they spread, they comingle! Also, you can use similar arrangements on a mirror's edge too! If you need a diagram, I'll email one later, but this has to be the cheapest way to combine beams! -Glenn
The choices are endless, there are things to remember though.....The rangefinder cube in my picture no longer works because I handled it too much and it caused bacteria to grow between the two prisms, it is now opaque to blue hence buying the dichro shown in the link....By the way that seller doesn't list paypal as an option but if you ask him nicely he will do it for you