Most of us see what you want to do, run separate running lights and signal light filiaments on the rear like the front. The down side to that is the two fillaments have a common ground creating a "sneak leakage path" while doubling current draw adding two extra filiments (one on each side) in parallel, possibly damaging your expensive TSSM at worst or at least cause diagnostic code to key up.
Use the isolation module...it is safer than replacing the $300 module plus dealer TSSM programming and marrying back to your ECM/ICM, labor and downtime. It is your bike, you can do whatever you want, myself I know what I am doing when it comes to electronics, and use 3 dimensional thinking to get the job done. In this case, since you do not know if HD adds isolation circuitry when they add light bars and such, just "bridging" existing circuits in parallel seems simple enough, but you do not know what logic/driver pass elements are inside the TSSM and is not worth the risk by just throwing hardware in to see if it works...JMHO.
Same goes with adding trailer wiring, off course you could do it yourself, but if you can obtain a "Plug & Play" kit with a guarantee and no cutting, isn't that what we want for our NEW, IN-WARRANTY possessions?. HD dealer that you brought it from might offer a discount to install it for you as well...such a deal for what is likely a common upgrade they do all the time.
