The pistons and chambers were matched at the time they were built so you won't want to mix them up. As the other guys have said clean them facetiously, including making sure the main carb body is clean and then see if they move freely. The three screws that hold the chamber on to the main body should only allow one way of fitting them.

Make sure the needles are straight as well, both how they are inserted into the piston bottom and the needles themselves.

Let us know where you get to. Be very-very careful not to try and sand or take metal off the piston to allow free motion, as said previously they are supposed to be and need to be a precision fit. If you absolutley feel the need to polish them I'd suggest that at most use 800 grit wet/dry sandpaper lubricated with oil.

You'll know you have it right when the pistions fall evenely and both make a nice clunk when they hit the main body.

Let us know where you get to!