Maybe we should define first what you mean by "manage two separate computers"?
If you want to control the Raspberry from the N900 the former needs some sort of system installed in the first place to process the commands that come from the N900. Since the Raspberry is supposed to support Debian and the N900 already runs Debian (sort of) that would be the logical choice.
Then we need some way to create an interaction between the two devices. The most commonly known way surely is VNC. x2x or Xpra might result in better performance but just like VNC they won't actually eliminate the barrier of still having two separate devices that run their own operating systems.
So what actually remains of the integration of the two devices are three things:
1. To use the N900 as a mouse and keyboard for the N900, which is only a stopgap solution since an actual mouse and keyboard will always be more comfortable.
2. To use the N900 for network connections. This can easily be achieved by attaching the N900 in PC suite mode to the Raspberry running its own completely independent Debian.
3. To use the data we have on the N900 which is a simple mass storage connection.

I was hoping to combine the strong N900's CPU (the Raspberry's will be weaker) with the Raspberry's strong GPU (it's supposed to be able to decode 1080p) and maybe even the combined RAM. Unfortunately that doesn't work at the very first step since Xpra already adds more overhead than the systems we're talking about are able to handle. And honestly Fabry's results don't seem more promising either.
So there's nothing special I see left here unless I've missed some important feature that the N900 might add to the Raspberry apart from the three I've mentioned above.
...or creating an artificial, unneeded integration just for the sake of doing it because as N900 users we want to integrate it everywhere. The Raspberry will work just as fine (or likely even better) without it.

Then just boot the Raspberry connected to a TV and connect the N900 containing your pictures. There's nothing magic or new about that.