actually it will make you INCREASE backpressure making you LOSE HP.
less backpressure = more exhaust gasses are still in your combustion chambers = less combustion = less power.
that is unless you have a custom header/exhaust manifold designed for dual exhaust, in which case you would reduce backpressure and increase HP.
some people say less backpressure will make you lose bottom-end torque but i've seen dynos that say no such thing.
bottom line: two mufflers may look good but most of the time it hurts performance.