WOW, somebody re-wrote the book and done got into some electrical/electronics I've never heard of.
Paralleling two eight gauge wires from the rear to the front of the boat is going to do nothing but make ONE cable slightly smaller that if you had run one piece of 6 guage cable to start with.
Circuit breakers should be at the battery end, not the TM end to keep from burning your boat up if they happen to short or have problems. At the TM end, they are only going to protect it if the TM itself has a problem. They are not resistors so they should not be limiting anything.
The only real limiting factor to the whole setup, provided it's not a large enough trolling motor you should be using larger than 6 gauge wire would be the single piece of 8 gauge wire you are running to the TM, Then it's going to be working like resistor and over heating with the more restriction it's causing. To the point it could actually melt.
Not sure where the voltage dividing and because it's two wires instead of one it will only draw half the current and that will let it run twice as long stuff is coming from.
The TM is a load on the battery. The harder is pulling, the more the load it's going to place on the battery. If the cable between the load and the battery are not large enough to handle current that load is drawing, they are going to start creating a resistance to the current and start to heat up. Heat is wasted energy. At the same time, as the resistance too small of a cable increase, it's going to create a voltage drop at the load/TM so performance is going to drop. However, the lower voltage and drop in performance is not going the increase battery run time because the heat being generated is wasting that.
Then, what since does it make to buy a 80 pound TM to have it perform like a 60 pound TM because you are running too small of wires to it.
If anything, you should be going to a larger 4 gauge or even a 2 gauge TM cable (which is what I run) for all battery connections and up to the TM. This limits the amount of wasted energy from heat, lets the TM perform at it's peak and provides the most efficient operation and longest run time.
If you are running a 12 volt TM and want more run time, upgrade to a 24 volt, A 24 volt motor is about 25% more efficient than a similar size 12 volt because there is much less internal heat being (wasted energy) generated. I don't have the numbers but I think a 36 volt adds about another 13% to that number. The higher the voltage the more efficient. That's why they run 600,000 volts down high tension power lines.