First of all you should understand why you have started both of these tablet. It might not be due to just because lack of appetite, you might have consulted with psychiatrist I think so. Venlafexine and mirtazapine both are antidepressant so you must evaluate yourself for symptoms of depression like low mood, loss of interest, disturbed sleep, irritable behavior etc. Increases appetite is just a side effect of mirtazapine. So you can take mirtazapine 7.5 mg morning evening, and withhold Venlafexine.