Will you ever get the 2nd hidden blade back?

Somewhere in the game, u find Leonardo, you will have to pay him so he can make it again for you, I think the price is 3900-4000 florins

What raixyn said but if you want the other stuff from leonardo you can find a white L and just sit on a bench and he will sell you some goodies

No you find him on a mission and he will give the double hidden blades to you for free because i have finished the game and gave me the double hidden blades for free

Just go to Leonardo and buy it back from him (it may get takin away from you on some missons

Yes you do

Yes. In Sequence 04,Mission 9. You Have The Choice Of either The Double Hidden Blade,Climb Leap Glove(Same As The Climb Leap Ability in Assassins Creed II),Or A Poison Dart Shooter.Replay The Mission 2 More Times after And You Can Get The other 2 Inventions. Hope This Helped. :)

It seems to come back on its own.

In Sequence 4 Memory 8, you will gain access to Leonardo and than you can purchase your codex items from AC2 (Second Hidden Blade, Climb Glove, And Poison Darts), since Leonardo is broke working for the Borgia; but to obtain all of them, which you could, you need to search out for "L" on the map. After obtaining all three items Leonardo set on the fourth visit, "You have all the I can give, you are on your own."

As ShadowPrince has said, you do have a option, although you do have to purchase the other upgrades after that. Leonardo dun like freebies :3

Yeah but u have to buy it from leonardo

Leonardo has to make Ezio Pay for them because he doesn't get payed a lot from the Borgia. You have to sit down on a specific bench which is marked on your mini map. Other things that can be purchased are the climb globe and poison darts.