That gun is a terrible idea. You can't use it on a bipod, or at least there is really no point in trying that, so you need a tripod or some other mount for it. And at that point a twin mount could do the same job, but much cheaper, not to mention that you can still use the individual guns separately if you changed your mind about this idea. And the only role a twin MG3 would make any sense is as a door gun for helicopters, which is quite a niche one. Developing a modern lightweight mount for that specific job would be a much more marketable idea, especially if they put in the extra effort to make twin mounts for the FN MAG and the PKM too.
And if they really wanted an alternative to a Minigun, then they should have turned it into a Gast-system: as one bolt goes backward from the recoil it slams the other one forward, speeding up the loading cycle, which also means it will make the first one reload faster, therefore the RoF is not only much higher than if you simply bolted the two guns, but the two guns are perfectly synchronized. I have no idea how fast two MG3s with a Gast-system would be exactly, but I wish I could build one such monstrosity just to find it out.