If you are running a pair of speakers for your HT mains, and an ENTIRELY different pair for your 2 channel yes you are fine using the 2 left over channels on the Rotel as a stand alone system with separate preamp and speakers. Of course this means your other 3 channels will be powered at all times and still physically hooked to your other 3 speakers, you can turn off the denon to kill any noise as that is the control pre for those other 3 speakers, but you will still have 3 other speakers getting signal at silence while using your 2 channel system and if you left the denon on while the metis is running you could get essentially noise or crosstalk, but not likely.
The real question here is do you plan to use the same sources? Meaning your DVD, or whatever running thru the metis and the denon? Because this will pose an issue, unless your just planning to use a cheap RCA splitter or your going to run your CD / DVD player with the digital out to your DENON processor which I recommend, and use the analog Jacks of the DVD player to feed your Metis. This of course depends on the sound quality much more of the DVD or cd source being shared. Of course if your doing Vinyl or Dac, or music server this will vary of optimal setup for you, without knowing what you are ultimately trying to do or share any sources at all nobody can make a recommendation.
In any case if you are trying to share several sources at once, you are better off getting a preamp with a true HT pass thru connection, not sure if the metis has this, maybe its an option to install, or if it has it than this is the way to do this all together if running multiple shared sources between the denon and metis.
The one other small risk at this is having two preamps into one amp could up the odd's that you get a ground loop hum, if you don't already with just one hooked up. And if there is a ground loop it will not necessarily go to silence even if you do shut off the preamp not in use. This will take on a whole other set of rules to eliminate.
Good Luck
