Hi Kipper, I think it would be really cool if you made this using two 25 pin sockets, and either removed or bent in the middle pins of one of them so two 9 pin joysticks can be plugged in. I have experimented with this myself and it makes a very neat adapter because it can fit inside a cheap plastic clamshell case and costs less to make since it only has two sockets instead of three.
I think there may even be one spare pin that can be used on the parallel port for a second fire button on the third player joystick, although no existing software supports it yet, if you connected this pin anyway it would give us programmers an opportunity to use it in our new games. I looked into this years ago, but I have lost my text files explaining it.