You shouldn't mix stims because your mental health can be on the line. Serotonin toxicity and psychosis are possible outcomes of mixing stims and while few people on the boards would report psychosis, a few in the past have reported mild serotonin toxicity. Symptoms include dry mouth, vomiting, dizziness ect.
Besides fat burners are seriously overrated. I have tried a number and they can't compare to proper diet and cardio. Unless you have those essentials in and are currently losing weight the stims won't do jack.