It's not so much twice a day that you need to worry about, it's more about the total workload you are imposing on your system. Take someone who does bench and squats on Monday. If he does bench in the morning and squats in the evening changing nothing else, then there is no increased workload. If he does both exercises in the morning and the evening, he is doubling his workload.
The question becomes, are you going to increase your workload? If not and there is no compelling reason to split the workout other than for arms and such - I'm not sure I'd bother. There are benefits to training focus since you are less tired and able to perform better. This is similar to the bulgarian method applied in olympic lifting - they spread their work throughout the day and are able to perform better and focus better in each exercise - and this is important for maximal attempts in skill related movements.
Your ability to handle a given workload has to do with your conditioning and genetic (and enhanced) ability to recuperate. People here talk about overtraining all the time but they worry about it on a body part recovery basis which is why there are so many 3 day split, 3 day per week workouts running around (which I think are garbage and you'll see why). The bottom line is that recovery from a bout of weight training takes a damn long time, well over 1 month for complete tissue remodeling (nice buzz word, look it up). You are always training in a non-fully recovered state. The issue becomes when the accrued deficit exceeds your ability to allow reasonable recuperation. This isn't a bodypart thing, this is a system thing. Too much load on the system and you wind up walking around like a zombie falling asleep on your feet and unable to put much effort into anything - that's overtraining (not just your chest being a little sore).
Indicentally there is no correlation between soreness and growth/strength increase among trained athletes. Soreness is indicative of applying a load to which the body is unaccustomed to (this is why the 3 day split where you do a single movement once a week and bomb the hell out of a bodypart with all kinds of useless garbage after the compound movement gets people sore). Now you might think an unaccostomed load means you will grow from it and that's called supercompensation. Unfortunately, the body doesn't work like that in the short term and certainly not workout to workout which is why an experienced lifter doesn't make much progress on such a system without drugs. You won't find pure programs like this used by the best coaches and athletes and you won't find their athletes sore (but they do manage to grow them very well). No one is using a 3 day split and applying supercompensation workout to workout. They apply loads in waves culminating in overreaching and then deloading. It is the progressive increase of workload over time balanced with periods of adequate recovery that forces adaptation. This is the basis of dual factor theory which is fairly common. You will find that the general BBer extreme supercompensation programs with their focus on training to failure are pretty much rejected worldwide among experienced coaches and researchers. The BBing world is pretty alone other than an occasional HIT afficianado who would be better served by being slapped upside the head with a 10lb plate.
If you are interested some good links are here:
This is probably the best thread you can read anywhere on the planet - the author is one of the smartest and best coaches in the country. It is worth skipping the gym to read this - you will be paid back 100 fold:
http://forum.mesomorphosis.com/showthread.php?t=12
Other good threads are stickied here (along with the first one):
http://forum.mesomorphosis.com/forumdisplay.php?f=3
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