What exactly are you trying to accomplish?
In general this is not necessarily something that will actually improve the sound at all, you will get a minimal at best increase in sound from the laptop, and will get phase problems acoustically from two sources in the setup described above. One from time differential between the laptop speakers and the stereo external speakers, and two from mixing a stereo signal down to mono, which may or may not have phase issues in the recording(Recent recordings don't be surprised if they do).
So while on occasion there may be a reason to do this, it is very rare I see a good reason for doing this type of setup. So what exactly are you trying to accomplish with this?