Can you take sertraline in morning and mirtazapine at night. Cheers...

If prescribed, yes. Sometimes psychiatrists do use this combination, especially when someone is having a big problem with sleep along with anxiety/depression. If prescribed, your doctor will be keeping a watch on the doses and any potential signs of serotonin overload. Generally mirtazepine will be at a lower dose than the zoloft (sertraline). If doing this on your own without your doctor's knowledge, please don't.
Serotonin syndrome. I would advise against using both as there is a risk of serotonin syndrome - which can be dangerous -have you spoken to your psychiatrist ?