There is is how you make a two headed pen.
This is before modifying.
This is in between.
This is after.
This is junk.
it is pretty self explanatory but I will tell you in more detail what you do is you take it apart and you put the ink cartridge and the adapter where the plug used to be (you might have a bit of trouble getting the ink cartridges next to each other but you can slightly bend the barrel to get one next to the other.