For starters, I’d suspect you want to add a socket to the char skel/mesh hand… then attach the weapon mesh to that socket. That should put the weapon in the hand (in the component part of your char BP, just drag your weapon in so it’s part of the BP… then in code (constructor or weapon change event, whatever), attach the weapon mesh to the socket…)
Not sure how you tell them both to move at once… anim notifies might work, or you might have them both in a montage (though, I’d think you’d need one for each weapon, and that might get tedious). I’ve not gotten that far in my UE4 learning adventures.