The difference between a stack decoupler and a stack separator

Docking clamps have very little shear strength, so I suspect if we just changed the decoupler into two docking clamps, your cockpit would fall off during the ascent.

It's a bit more complicated to replace decouplers with docking ports; decouplers just fire off of your current stage, and stick to the stage you might want to return to. That doesn't work so well when you want the decoupler to detach from BOTH stages, leaving the docking port free for later use.

So what you want is:

• A docking port on top of your fuel tanks.
• A decoupler on it, upside-down, so your lander detaches from your extra fuel.
• A decoupler on top of the other one, right side up, under your engine, that will fire both decouplers off of your lander, and into space somewhere.
• A docking port somewhere on your lander. On the nose, if possible. Anywhere else will require you to dock on your side, which will make controlling it more difficult.

Pictures of this will be forthcoming shortly.

This setup will allow you to take it back to Kerbin (if you have enough fuel remaining), and leave it in orbit for future use.

I can't tell what you're normally supposed to do, but I installed the Firespitter pack first, and then the B9 Aerospace pack. I chose to overwrite all existing files and it's working for me.