Let me preface by admitting I'm no subject matter expert, except to say that I've successfully lost and kept off alot of extra weight myself...
I believe the current trend is to overanalyze everything, because of you come out with a "new" idea that claims to accelerate the process in any way, you can make millions of dollars selling books and diet plans. I believe the bottom line is, for anyone eating anything approaching a "normal" diet (i.e., things like atkins may permit a greater spike up front), that you eat so much food a day that provides your body with a certain amount of energy. You do so much work a day, for which that energy is used. This is includes everything from simply keeping your body alive, to things we do to intentionally burn some of this extra energy. On a given day, if you use more energy than you consume, you body will look to its energy reserved (fat). If you consume more than you use, it will be stored as reserves.
From an energy burn perspective, it should not really matter whether you do 2 30 minute workouts a day or one 60 minute workout. If you are doing the same work, you are using the same amount of energy. What they're likely refering to when they say you don't burn any fat during your workout until after 30 minutes is the different ways the body stores energy. Long term reserves are stored as fat, energy meant for more immediate use is stored in blood sugar.
It probably takes 30 minutes of cardio to deplete the stores in your blood sugar. So, until that point, your body is not turning to fat for energy during your workout. However, your body still needs energy to sit down in your car, drive yourself to work, breath, pump your heart, etc - while you are not "working out", you are still consuming energy, albeit likely at a lower level. At this point your blood sugar will be low enough that your body turns to fat for these sort of functions, until you consume more energy (food).
If you do the same 30 minutes again at night, and deplete the energy stored in blood sugar, then you will go through the same cycle. Yes, your body still needs energy to maintain itself and burns calories while you sleep.
If you do one 60 minute workout, then yes, your body will more likely need to turn to fat for energy during the workout. However, once you eat again, your blood sugar levels will remain more constant for the remainder of the day, and your body may not need to turn to fat for any energy after this.
Your weight coach was more than likely refering to strength or performance training when he suggested periodically altering the intensity and length of your workouts. Yes, you will tend to plateau in any performance goals unless you do this - but it should not really otherwise effect the amount of work your body does, and thus so the amount of energy it needs. The caveat being that the more lean muscle mass you have, the more energy your body will need to maintain it. A 200lb guy w/ 10% body fat can eat more than a 150lb guy with 10% body fat and still maintain his body fat level.
Bottom line, I'd say stop overanalyzing it - if you just want to lose some of the fat, eat less and exercise more, and it will come off. And do it with a regimen you can maintain for the rest of your life.