If you have a pair of front speaker s and a pair of rear speakers, why would you want to put any two hot (+) wires together. You should have a separate hot wire for each of the four hot wires coming out of your new stereo. If you have doubts which wires go to which speakers, use a small 9 volt battery and attach it to the wire pair in question. You will hear a pop as you attach the wires and you can determine which speaker this sound is coming for, noting which wires created it. You should also have 4 separate ground leads and only one will work with the associated hot lead that you are trying to locate, position wise.
So, yes, you could in fact tie the front and back speakers of one channel together, but you loose the front rear fader option.