One bulb is fine. If you really wanted the other you could just modify the harness. I dunno, guess you'd need another set of diodes then huh?
I saw someplace the diodes create some resistance and thus the taillamps are a bit dimmer, but they seem to be fine on my subaru. Can't be any worse than these bozos that put tinted lenses on their taillights just to be cool.
They didn't have a setup like that for the Subaru (yet), I'm sure the HOnda is WAY more popular. So I had to make my own connections with the diodes. Was really pretty easy, and there was room in the taillight housings for the connectors and diodes. All worked really well, I'm very please with that setup. The last car (scion xb) I drilled the lamp housing for an extra taillight bulb. It worked okay but was kind of messy and mcgyver'd looking. The diode setup is a way cleaner setup.