For the first, you have to do enough of the Homestead missions and collect some of the treasure chests scattered around the cities in order to unlock the Double Holster recipe. Obviously, you can only do this as Connor.
Second, it will, but you can go back to any shop or even the basement and pick up your old weapon (going back to the shops works just like buying a sword the first time, but you don't have to pay for it if you already own it). Note that the stats are essentially meaningless, the only one that really matters at all is Combo and even that is rendered worthless by virtue of being able to counter and start a killstreak.
And... One more thing: I noticed that obtaining new equipment like swords and such alter the appearance of Connor, as in, you see it at his side and his weapons are holstered around his body, I suppose that the double holster for pistols will also show up?
What other things can you buy/craft that will show up on Connor? If you buy pouches for ammo and such, will it alter your appearance?
Yo Buddy, Still Alive? Rest In Peace - Our Beloved Dog Lukas (07-31-12) - Someday, We'll Meet Again On Rainbow Bridge
When you craft the twin holsters, you will see both pistols. Swords and tomahawk/dagger will appear on either side(I am of the opinion that he looks STUPID with a sword equipped. I ran two full playthroughs without ever picking up or equipping a sword...again, my opinion)
Pouches only increase the total ammunition to be held.
Some cinematics may show dual holsters even if you do not have them crafted or equipped, simply because it is put there to show he can have them, but the right holster will be empty unless you do have them equipped...
Also, at some point you can find a chest somewhere(i forget where) that has a pistol recipe that you can dual wield the pistol and each one fires two shots before reloading, allowing a total of 4 shots before reload. It's what I use and it is amazing, but I think it requires more endgame material. Honestly forget when and where I picked it up, and only found out it fires multiple shots per gun by reading the weapon description.