I've been following along and I can't think of a way to do it either.
Linking Zones requires a second Zone, as does a Zone Group. You can't play to a Zone Group anyway.
I thought about copying the existing DLNA Renderer Dynamic Zone, but that only allows you to copy a non-Dynamic Zones. (Copy via the right-click on Playing Now, Zone Add feature.) Besides, if two MC DLNA Servers are talking to the same device, I think that would end in tears.
Playing a mixed Playlist of 1xDSD and other formats, requiring the DLNA Renderer to switch DLNA Server for specific Tracks, would probably also end in tears. They don't always switch well, or quickly.
What would actually be required is functionality like Zoneswitch, which changed DLNA Server association with Zones. But again, that may not be a robust solution, depending on how well the DLNA Renderer handled that.
If the Denon AVR-X3200W had multiple inputs that could identify themselves as independent DLNA Renderers, then I think you could use those for this purpose. That would be something to look into for the Denon. It does have Wi-Fi and Ethernet network connections. As they would have different IP Addresses, maybe the DLNA Renderer on the Denon would be visible via both, and present as two renderers in MC. Again, I would anticipate control issues.
Otherwise, I think the solution is to convert all DSD and other formats to PCM. The quality is going to be good to excellent, and it will be a much simpler installation to manage.
Of course, if the Denon could play all DSD formats, that would be easy to set up in one DLNA Server.
Actually, as the MC DLNA Servers decide which files to convert and which to send to a DLNA Renderer untouched based on the file extension (I'm pretty sure it is file extension, and not the [File Type] tag), then you could use one extension (file format) for 1xDSD files (say DSF), and another for other DSD files (say DFF). DSF is the preferred format by most users, and I think DFF allows less internal tagging. Not sure. But you may even be able to get away with using DSF files and just changing the file extension. You may need to change Mime Types for the DLNA Server, so it tell the DLNA Renderer how to play the files correctly. You may even be able to invent your own file extension for files you want MC to convert, because the DLNA Renderer would just receive PCM for those files, and they should play fine without the Renderer needing to know how to decode the file it receives.
Maybe try leaving 1xDSD files with an extension of DSF, and for higher formats, 2xDSD and above, change the extension to DSFx, then set the DLNA Server to convert DSFx files, but not DSF files. See if that works. You may still need to add a Mime Type to the MC definitions for DSFx, but possibly not, and that isn't hard.
Give it a try.