1. If you are talking about local multiplayer (as in two players using the same keyboard on one screen) then it's pretty straightforward. Just give the tank the desired movement behavior and then set both tanks' controls to different keys. If you have different tank images on different frames then set the animation speed to 0, then you can either set the frame in the properties bar or via the action: Tank - > set frame.
(if you want to tackle online mutliplayer then just really don't.)
2. If you have multiple turrets and multiple tanks to pin them to, you need to "tell" C3 which ones belong together.
I think the simplest way to do that is to create the turrets in game as opposed to doing that beforehand.
Basically it would go something like this:
* Tank: spawn another object (Turret)
* Turret: pin to object (Tank)
Explanation: Since you create the Turret inside the Tank's "on created" scope, C3 will "take into consideration" the particular instance of Tank and Turret that should be pinned together.