Let's imagine you're not limited by time/work, therefore you can work out twice a day, for example once in the morning and once in the evening (big time gap between those two work outs).
Is this a good idea, or a bad idea? The aim is to gain strength and muscle, not lose fat.
It's perfectly acceptable to work out twice a day, so long as you're not working out the same muscle group twice. For example, in the morning you could do upper body, and in the evening you could do lower body.
The purpose of strength training is to tear down the muscle fibers and then supplement with protein. The protein breaks down into amino acids and repairs the damaged muscle fibers. Protein synthesis is optimal during sleep, so unless you sleep most of the day between workouts, training the same muscle twice in the same day is actually counter-productive since your goal is to get the amino acids into the muscle for hypertrophy and your body just won't have enough time to allow that to happen without proper rest.
Over-training is bad news. The muscles become fatigued and lose their ability to respond and handle load, and so the forces are placed on the joints and connective tissue.
That really depends on your particular goals. Football coaches are notorious for their 2-a-days, but those are there to build stamina at the current strength level--not necessarily to build more raw power.
It's a healthy habit to cycle between improving your raw power and endurance. So it really depends on your current goals. If you are currently just starting out, I recommend increasing your raw power before you work on your endurance.
For further information I would recommend reading "Practical Programming" by Mark Rippetoe and Dr. Kilgore. It helps understand the the concepts of stress (work) and recovery, and how your needs change as you progress from beginner to intermediate and beyond.
One more thing to think about... For general heart rate conditioning (trying to get resting rate below 50 beats per minute for example) it's total time that matters. If it's easier to do two sixty minute workouts than one 90 minute workout, so be it. The same is true for improving a specific workout. If you want to do 100 pistils, doing dozens of smaller set throughout the day can beat one or two large sets. (This is referred to as greasing the groove) That said - for other sport specific activities, it can be better to group the workouts. A lot also depends on your recovery time.
