If the lasers you are using are not sealed tubes, the easiest way to combine them for doubling the output power would be to 'merge' the two laser heads into one. Basically, you would remove the HR from the first laser and the OC from the 2nd laser. Place them end to end, and you have ONE laser with twice the power. The two heads could be placed side to side, using two 100% mirrors at 45 deg. to 'bend' the beam from head to head in the rear. This technique is commonly used in industrial lasers.
If your tubes are sealed, or if the above approach just isn't practical, things get considerably more complicated (and a LOT more expensive!). You would need both beams to be polarized (a dedicated optic each) and then merge them using a combiner/splitter (VERY high cost). This would give you one beam (so to speak, it would still be two beams just sharing the same path and not interfering with each other). However, you could only pull this trick once. You would not be able to 'mix-in' any more beams.
One last word, the cost of the optics would almost certainly exceed the cost of just buying a 200watt laser in the first place.