You would need to put a Jamma harness in the cab along with jamma adapters for each board and use Mikes 2 in 1 switcher or the 6 in 1 switcher from Jammaboards. There is a 2 in 1,like the 6 in 1 out there,but it is a POS.
You also need to hack in a switching power supply or make a power harness to go from the DK power supply to the jamma harness.
Then you need a connector to hook up the video from jamma harness to the monitor and also an inverter to flip the video,if you have a sanyo monitor.
Mikes is more expensive @$135.00 and powers both boards at the same time and the jammaboards is cheaper @ $99.00 and powers only the board in use.
The jammaboards switcher need the -5v mod to get sound from the DK & DKJR boards but that is an easy mod.
They will also do the mod for you @ $5.00 a slot,so that would be an extra $10.00.
Either way you go it will cost a few bucks and be some work.
It is nice to not have to pull the cab away from the wall to switch games.
Here is my DK with 6 boards and the 6 in 1 in it.
The 6 in 1 is under the coin box,so it can't be seen in the pics.