I'm far from expert.....but.... as others have said its very 'difficult' to both build additional muscle and loose weight at the same time. To burn fat effectively you want to work out at medium intensity (65% of your max heart rate). When you work out at higher/anaerobic heart rate levels your body switches from burning fat to burning glycogen.
My best weight loss was brought about by doing split workouts - I'd go to the gym in the morning and do some miles on the elliptical or treadmill and then go back after work to lift weights. It made a huge difference compared to going once a day. Of course, it takes time to drive to the gym, workout, have a shower.....so it wasn't a viable option as a long term strategy, but it did get me through a plateau where I wasn't seen obvious progress.