Adding second d200

[Hifi_swlon]

…. I have not found this information on the website, perhaps my error.

On Devialetchat?  You might not have looked very hard

All the information you need is in post #2.  You need the appropriate cable and configuration - that's it. No hardware changes or removal of anything.

To make a D400 you can either use a 200 companion model, or another 'full' D200.  The latter is usually more expensive but since you seem to have a deal is probably the best plan.  It's also good for resale as you have two full D200's if the time comes.