Hey, you wouldn't be the first to call me crazy, and the more I protest, the more it seems like an admission that I am completely off my rocker....
Seriously, though, the reason I ask is not to be a smartass, but because it is not that difficult to set up separate HT and two channel systems so that they both use the same main speakers, and I was just wondering whether you had considered it. I wholeheartedly agree that a dedicated two channel system sounds better than anything I have heard in the HT realm when playing regular old music, and I wouldn't encourage anyone to listen to music on a HT system if they are inclined not to (not that there is anything wrong with that, as they say...).
In case Im not making any sense, (an forgive me if I am being overly simplistic), allow me to complicate matters further. Say youve got a two-channel system consisting of a CD player, a preamp, an amp and a pair of speakers and you are looking to expand it to include a HT setup as well. To do so, you get a HT receiver and three extra speakers (center and surrounds). The two-channel setup remains the same and, when listening to music, thats all you need. Now, you can add on the HT gear as follows run you HT source (presumably a DVD) into the receiver which, in turn, powers the center and surrounds, as usual. However, instead of running the main speakers from the speaker outs on the receiver, run those channels from the main L and R preout connections into the two channel preamp (into either a HT bypass loop, if available, or any spare input otherwise).
If you have a HT bypass loop on the preamp, using this setup is simple. Simply turn it on and the preamp will pass the main channel info from the receiver on to the amp and the main speakers and allow you to control the volume from the receiver. If, instead, you are running the mains into, say, the AUX input on the preamp, youll have to set the volume on the preamp to a set level (such as 12 oclock), balance all of the channels using the receiver (which youll only need to do once), and then you can control the volume for all of the channels from the receiver as well.
If I understand correctly, you already have all of the gear you need for this (plus an extra pair of speakers). (Assuming, of course, that whatever you are using as the processor/amp component of the HT setup has preouts for the mains and the ability to balance the volume level of the main speakers independently from the other channels). For my money, this provides the best of both worlds -- you have the dedicated two channel system plus the whistles and bell of HT when you want it.
Phew, long answer to a simple question (that wasnt even the one you asked, either). In short, if the logistics of having two pairs of speakers in the front are troublesome, dont bother -- you only need one. You should be able to set it up so the towers are pulling double duty without compromising the two channel setup at all. Again, maybe Im missing something, but this may be an option worth considering.