This is a good one. Several studies have been done on this very topic. The results are as most of you thought: No significant difference in the cardio-vascular benefit between the two. Although if warm-up and cool-downs are included the split routine takes longer. There was also a slight edge towards the longer work-out to effect changes in the aerobic metabolism. My guess is that if the studies were longer in duration the once a day would have probably shown superior. Keep in mind these studies were done at low to moderate levels of intensity. My soap box comment: Examine information from magazines and infomercial with a discerning eye as they often misquote and misrepresent published literature by taking it out of the context that it was conducted. I have experinced this first hand. Can you say Fast abs, Ab-isolator or Ab-energizer! Watch 20/20 in the next 2 weeks and you will see what I mean, if they air what they should.
A couple of points though:
In general the minimum aerobic work out length should be 30min. If you plan on doing high intensity i.e. a moto, I would recommend a 10 minute gradual warm-up and cool down with around 15-20 minutes in the high intensity zone.
If you are doing intervals this doesn't include the recovery periods. A good high intensity work out usually takes at least 60 minutes for a well conditioned athlete. That is 1-2 times per week certainly not 2 a day. The key is that you spend a total >15 minutes at the high intensity to reap the most benefit. For example treadmill training: 10 minute warm-up @ 5-6mph with 1% incline. Next increase either the incline or the speed or both to a level that you can accomplish with some effort for at least 90 secs. I prefer at least 2 minutes. This will take some trial and error. After this high intensity reduce to warm-up level or lower for the same period of time. Repeat intervals until you have totaled 10 minutes at the high intensity, that's a pretty good start for a newby. Aim for 15 minutes at the high intensity by adding an interval a week. Once you get to 15 for 2 consecutive weeks, up the intensity. Substitute treadmill for whatever floats your boat, but use the same guidelines.
Mix this in with moderate intensity and low intensity recovery work during the week. My advice for training for a moto is do it on the bike if you can. Get to the track or woods, warm-up and than try to ride 10 laps or 20 minutes at a good clip. You can't get more specific than that. If a track is not available try as suggested: mountain biking, elliptical training, rowing machine(watch the knees) combined with circuit weight training. No rest between sets.
bbbom gives good advice in finding your weak points and stress those areas in your gym work. Ask yourself where you fall apart during a moto, legs, arms, lungs, all of the above and than you can individualize your plan. I can tell you from experience that the first thing to do if you have not done so already is develop a base level of fitness. Do so in a easy progression with basic movements and training. Integrating intervals, speed work, and complex cross training from the get-go will often lead to failure and injury. You will notice and immediate impact on your riding from this initial training and than you can fine tune to your needs. Riding when fit makes the experience, whether racing or play-biking, that much more enjoyable.
Good Luck, Keith