Maybe maybe not.
Student Mechanic where do you get your ideas from? You know even less about motorcycles than you do cars.
Hi Pro motorcycles have one pipe because the entire exhaust system is tuned to the upper power band. Bike engines have to be small and light and powerful, and the faster you can spin the crank the more explosions per second you can get, thus the more power, so designers can only meet the power demands on a small engine by spinning them faster and designing everything accordingly. At these higher RPM's the exhaust system can scavenge if the system is tuned for it, in short not only is there no back pressure there's actually a negative pressure there in part of the exhaust cycle that literally sucks the exhaust right out.
Car exhaust systems are not tuned, at least not the entire system. The headers can be but the cat-con and muffler prevent the rest of the line from being tuned, and in addition people and municipalities would not tolerate the loudness of a tuned exhaust. Bikes can get away with this because there's fewer of them and people expect motorcycles to be noisy. Also, car exhaust has to be dumped at the rear bumper or you can suck it into the passenger compartment, thus the exhaust line has to be much longer than optimal for tuning. Finally, the benefits of a fully tuned exhaust vs just tuning the headers aren't that much better.
Because of this, the better flowing the exhaust is, the less backpressure. Granted that 2 smaller pipes flow less than 1 bigger one, but 2 bigger pipes flow even better than 1 pipe of the same diameter. So while putting dual exhausts on a car may not gather that much more power, it's always better than a single exhaust, assuming equal sized pipes.
Ted