Some power supplies can be linked to achieve a gain in output. This is common with some processes (e.g.: SAW). You probably would want to check with the manufacture before doing so.
Why not move the supply closer to the gun and extend the primary supply line instead. The primary voltage is higher thus it can get the power through a smaller size wire (saves money). If you get a little voltage drop on the primary side you (or the regulator in a CV supply) can use a little higher setting to compensate.
Bill
One of the most common problems with welding a long way from the power source is weld cable that is too small. For example a weld circuit of 400' in total length to weld at 200 amps would need to be 4/0 weld cable. This size is needed to reduce the voltage loss that will happen at this distance. As to the question of parallel or series. Parallel will double the amperage output, series will double the voltage output. Hope this helps.