you can do a few thing. Im sure someone makes something to Combine then to use on one Speaker.
If this is how its working... The speakers are Dual voice coils and they send base to one and Mids to other. so you just buy Dual Voice coil speakers.
If it works like this... I just hook up the Base line because thats what the 6x9'* are good for in the rear base.. i dont need mids in the rear. fronts can do that.
Get another set of speakers just for the mid or base line.
But i think that that have those lines going like that because there is a cross over maybe in the Amp
Im going to check with a local Audio store my brother is friends with the owner and see what he thinks and also check with my other brother witch use to be Really Big into car audio professionally