First, in clinical studies, Lexapro is weight gain neutral, meaning that just as many people gain weight as lose weight. With that said, it is impossible to say whether the drug itself actually causes people to gain weight, and if it does it is unknown as to why which further complicates the matter.
Second, you indicated that the the person had depression. Adderall is not recommended for people with depression, however, new studies have shown Adderall to actually be effective. Now the second part of the question that states that you have obsessive thinking. If this is the case, then I would definitely be very wary of taking Adderall, since Adderall is likely to increase this behavior.
Third, both Adderall and Lexapro act on the same neurotransmitters. With that said, they are still very different drugs, but do run into the potential of counteraction each other. For instance, Adderall works primarily on dopamine, but has also been shown to increase serotonin as well. Lexapro works mainly on Serotonin. Now most SSRI's also will to a lesser extent increase dopamine and norepinephrine. If you are to mix Adderall with an antidepressant, Lexapro would be the best one to do so with, since studies show that Lexapro is cleaner and acts on serotonin and does not seem to increase the other neurotransmitters.
Fourth, it is unlikely you will achieve the results you want. The truth is, Adderall works to decrease the weight mainly by suppressing your appetite. It also increases your metabolism, but this increase is negligible and is not likely to make any significant difference in weight. Also, you will notice a decrease in appetite at first and the Adderrall will work initially; however, over long periods of time, Adderral is not very effective in managing weight as your body becomes used to the drug.
The best way to decrease your weight is through exercise. You should be doing exercises that elevate your heart rate to over 130 bpm, at least 4 times a week for 30 minutes - 60 minutes. While any exercise will burn calories and help, exercising of this intensity is the only real way to release endorphins which naturally increases both your serotonin and dopamine levels which will help reduce your need for the antidepressant and make your existing dosage more effective. Also, this level of exercise will elevate your glucose levels, thus naturally suppressing your appetite. And finally, even if the Lexapro has decreased your metabolism, this should offset any decrease and set you on the path to weight loss.
Of course, not everyone can manage such a routine; however, it is the one likely to give you consistent results. In any event, you should speak with your doctor to weigh the pros and cons, as Adderall is a stimulant and a Schedule II controlled substance. Adderall has done many wonderful things for many people, especially in the area of ADHD; however, it also has the potential to do many negative things as well and has a higher potential of addiction than most other drugs.