The best of both worlds is a 2-1 like D&D with ghost pipe if you like the look of two

 

Bingo - Fat Cat is a great pipe and if you are going bonkers with the build, Borzilla is your friend - Then the Ghost Completes the look

See attached of my old '15 - FatCat w/Ghost Pipe

 

Is it possible to just add a 2in1 header to my existing Pipe?

 

Doubtful, hence the reason I included a Pic of my old bike - Look how far back the Head Pipe goes before it connects to the Can....I believe it is a tad longer than a standard 2:2 or 2;1;2 head pipe - MAYBE V&H Pro Pipe could be a fit for your V&H Can but from what I know, when you go 2:1, you need a can to support that - Your Current Can is designed for only 1 side of the exhaust - I may be wrong.....as it happened once back in '92.

 

A 2-1-2, like the FM head pipe isn't a bad way to go either , just getting read of the stock CAT will do you wonders. I'd also but the baffles back in , they will greatly improve your low and mid end , where you must likely want to be . I have the FM head pipe w the 4" Reinharts and love the nice low rumble and performance from them.

 

+1 on a D&D 2-1. I'm not fully understanding your question though. If you want to run a 2-1 you will have to remove the catted Harley 2-2 head pipe and replace it in favor of a 2-1 head pipe. 2-1 pipes will come as a full system with the head pipe and muffler as far as I know. Some companies even offer a "dummy" or "ghost" pipe to complete the dual look. I know in regards to a D&D, if you were able to put a V&H muffler on it, more than likely it wouldn't look right because a D&D is built to be a 1 and done system. If you did get your muffler on a 2-1 you would probably want to put a baffle in it