It is just a matter of having the proper holsters and slings to make it happen.
What would be the point? So he could fire more shots without having to reload? What stopped him from firing everything he had? Or did he fire them all? (I haven’t been following this story much).
And if so, then changing the magazine capacity limit isn’t going to stop somebody who’s prepared to wear 4 different guns.
Then again, having laws isn’t going to stop somebody who’s prepared to break the laws anyway.
What stops a shooter who’s firing at innocent people is being dropped by the gun of somebody else.