It probably depends on how hard you are hitting it, I would imagine. If you are hitting it as a secondary muscle twice a week and never primary, it's probably getting a much lighter workout than you actually dedicating time to it. Personally, I lift with supersets after I do primary "big" lifts like bench/deads/squats/overhead press, where I'm constantly just hitting other muscles, so that when I'm done a chest day, my triceps are done for a few days no matter how much I want to engage them. I subscribe to the concept that no matter how well you do isolation exercises, your support muscles are usually helping. So I like working the shit out of those support muscles in the process, making my isolation exercises even more effective. A lot of super effective chest exercises still incorporate your triceps, so I definitely try my best to kill them during the process so that my chest has to work harder than normal on things like cable flies etc.
But yeah, it really depends on the relative intensity of the workout regarding how often you hit them during a week. I'm a big proponent of one heavy day and then 4-5 days of rest for that muscle group, but if you want to hit them more lightly and do it twice? As long as the overall work at the end of the week is roughly the same, I would imagine the results will be similar. I can't imagine that someone who isn't some genetic metabolic freak is hitting any muscle group twice in a week with harsh intensity like a single per week workout. I'm not even sure that's physically possible, since my tri's are legally dead in most states for at least 3-4 days after I abuse them.
edit - Though I am curious about the bicep thing. I know guys like Fletcher preach the guns, but I've found mine are pretty big for my size (not a big dude like some of the massives in this thread. If I bench 265x3 I'm pretty stoked as a 155lb dude, if I put 4xx on the bar it means I'm committing sepuku via barbell.) and I've spent probably 30 minutes in the last year working on them. I suppose it has to do with your adherence to isolation exercises or if you are compounds-all-day, erry day. I'm all about compounds mixed with isolation on major muscle groups, and I tend to just incorporate muscles into supersets. But biceps are usually sore by the end of the week and I've never felt the need to focus on them. Again though, it's probably more related to the composition of the workout.