The issue in your case:
Two different doctors prescribed them separately, which means neither may know about the combination.
Even if both medicines are safe together, it is important that one psychiatrist supervises the full treatment plan, so dosing and monitoring are consistent.
Next Steps
Escitalopram 10 mg (morning) + Mirtazapine 7.5 mg (night) is a known and usually safe combination.
But since they were prescribed by two different doctors, please inform both doctors so that one can coordinate and supervise your treatment.
Do not stop or adjust doses on your own 👍