wonderful job
seriously, you did very well on this. you probably got a little help from the fact that the 870 is so versatile. the military used to have the option of mounting an extremely short 870 to the underside of their M4's foregrip in the same arrangement that M203 grenade launchers ar attached to M16s.
seriously. the gun looks sick. even a professional gunsmith wouldn't have thought of this. combining a rifle and a shotgun has been done before, as i'm sure we all know, but slapping two pumps side by side -that takes GUTS.
however, if you REALLY want to make it look like something out of Doom, you could drill holes in a piece of thin sheet metal and have it cover the tops of both barrels like on a Mossberg 500, that would be downright stylish.
(1/8 - 1/4 inch thick would do it, out of somthing light weight, with a high melting point so it doesn't screw up the finish on the gun when you remove it to change barrels)
you should also make a full punch list of materials and step-by-step instructions for anyone crazy enough to try and duplicate this stunt, because somthing like this is worth duplicating. a gun like that, even if it wasn't so unique, could easily sell for at least $4000. (if the baseball bat used for the pump was signed by Babe Ruth or someone like that, it would significanltly increase the sale value of the gun with a corrisponding loss in resale value for the baseball bat. LULZ)
have fun vaporizing quail in the meantime