You get the 2nd Jack early because it's very unlikely you'll draw the 2nd Jack dead. With no other actions, Jack only draws 1 card and you're gaining extra Silvers into your deck, making collisions rare.
In contrast, you usually don't pick up the 2nd Smithy until later, because if you buy it too early they'll collide too often. A rough rule of thumb is 1 Smithy per 8 cards in your deck or generally 1 action drawing N cards per 5 + N cards in your deck. Obviously this rule of thumb stops working with engines, or when you mix non-terminals into your deck.
Odds are you can do something better than double Jack anyways. Jack + <something> BM is usually better than Jack BM, and sometimes you can get a semi-engine/Good Stuff deck with Jack on the board.