We'll be making here a pen that works like a regular pen but has two ball points.

Step 1: Supplies

So, what you'll need to make this:

wire cutters
two pens (try to find pens with a straight barrel, any other kind simply wont work)
an ink reservoir
some paper
glue

Step 2: Disassembling and Sorting

Take the two pens apart. Make sure that you have all the parts like on the picture.

Then sort the necessary parts form spare parts as shown on the picture.

Step 3: Bottom to Top

Now take one bottom cap and push it into the top part of the barrel.

If you have the right kind of pen this should go fairly easy in the start but become tough (because there is not enough room there).

In the end the top part of the barrel will snap to make room for that bottom cap.

To prevent this from happening you can sand down the groves on the bottom cap and possibly sand down the inside of the top of the barrel too.

Step 4: Assembling

So take the parts that we need and assemble one pen.

Step 5: Cutting to Required Length

Now take that spare reservoir and cut it like on the picture.

Step 6: The Bridge

Now use something to put into the part of the mechanism that is being pressed.

Why? Well, you would want to block that part so when u press the top of the pen the mechanism will still work.

If this doesnt work well (it didnt work for me), you can always do the thing that i did here .

Step 7: Assemble Top Part

Here is how i solved the problem in the previous step.

I modified one part of the other pens mechanism (spare part). I sanded down the edges so it could fit into the bottom cap.