Yes pluggin in an external speaker in the rear does stop the onboard speaker from working.
W5PKY gave you solid advice. Have a look here if you dont know what he is talking about:
Wiring Diagrams | Eminence Speaker
That is a page for subwoofers but the same still holds true.
More resistance = less power being pulled through the sound circuit in the scanner and probably a lower volume at each speaker compared to having just one.
Less resistance = more power being pulled through the sound circuit in the scanner and possibly burning up something. It may be able to handle a 4ohm load or it may not.
If you dont know how to add up the ohms by looking at the diagram, then all you have to do is wire it up or use the splitter and check the mono tip going into the scanner with a multimeter. That will tell you exactly what you have. Chances are it wont be exactly 8.00 ohms, 4.00 ohms or 16.00 ohms but it will be a fraction off. That is normal.
My advice is to go get a pair of amplified computer speakers from Kmart or Walmart, a mono to stereo adapter, and some extra wire. Plug the speaker plug into the adapter and the adapter into the scanner. Splice X amount of feet of wire between the two speakers so now you have one speaker (the one with the power button and ac adapter) where your scanner is and the other will reach to your other room. I use computer speakers on all of my scanners in the house and I never have to worry about not being able to hear it.