If you have an Accord V6, it comes from the 6-2-1 headers into the cat and then the single-pipe cat-back exhaust attaches to it with three bolts. Since it's a single pipe, power-gains won't be affected by going with a single exhaust or a dual.

The main purpose of going with a dual setup (besides looks) was to reduce the amount of sound created by the exhausts. With dual mufflers, you are able to gain twice the amount of sound-deadening surface area on the inside of the mufflers. Therefore, the only thing you'll get with going with the dual option is less decibels and a slightly different, lower tone.

Back in the days, if you can find it, they had dynoed four setups and realized that the dual option doesn't gain any more power than the single, so if you really do like the look (or want to go sleeper), go with the single.