If you have a button programmed for "External Speaker" you can run in Internal+External mode, where both head speakers and both deck speakers would function (great for extra volume), or in "External" where the deck speakers are the only functional speakers. With Internal only, which is what it sounds like you have, then you'll hear only selected audio from the head and unselected from the deck.
It's of note that only one head can control the volume of the deck speakers at once, there's a menu item/button press to toggle between Head 1 or Head 2 being the master volume control. You could wire two speakers at each head back to the deck, but you'll have issues loading down the amplifiers if you're using KES-5 speakers. If you ran an 8ohm speaker, you could run them in parallel as the radio supports a 4 ohm load. You couldn't (shouldn't) run KES-5s in parallel. If you have an inventory of old crap, some of the big Motorola speakers were 8ohm and could be paralleled.
However, you'll run into the training issue of changing control between the volume knobs. There's no function for unselect audio, so you couldn't dedicate a deck speaker to be just unselected audio, you'll have to have dedicated speakers per band.
Hope this helps. I run a KES-5 on each deck and a KES-3 equivalent on each head and in Int+Ext, since I have a very loud vehicle and I want to be able to hear selected audio clearly.
