At the end of the day, the products are all made to work in terms of minimum calorie contents with maximum nutrition. As LT and exante are so alike, mixing them for a couple of days should not at all be a problem. If you want to, you could always compare the vitamin and mineral contents of the two.
I know where you are with your 12 weeks, with me it's probably gonna be longer due to more weight to lose. But I will have to have a major break soon due to seminars I'm going to go to. Will try and lose as much as I can until then. And then see if I can make it a working solution/low carb eating programme for while being at the seminars. This in the past would have made me break my diet thinking what's the point, I'm going back to normal eating soon. But that's just pure self sabotage and I'm not allowing myself to do that yet again.