Do you have any details of which diodes these are? I am very doubtful they are actually 15W, as the most powerful visible single diodes are in the 7-8W range.
However, you can indeed combine the beams. If they are of different wavelengths, you can find a piece of dichroic glass which will reflect one and be transparent to the other. If they are the same wavelength, you would need to use a Polarized Beam Splitter (PBS) cube that combines the horizontally polarized component of one laser with the vertically polarized component of the other laser. In addition, there is the method of knife-edging, where you align the beams side-by-side using the edges of mirrors.
Did you buy some laser engravers from a Chinese site? These are typically stated to be 15 watts, but they are not. You do need to supply more information about what you have, specifically, to get our help.
You mean to tell us you bought a bunch of laser hardware, with minimal knowledge of the specifications, and no idea of how to incorporate it into your final project? Only AFTER you've made your purchase do you think to ask for advice? You don't think that's a bit reckless and hasty?