What will happen to me if I take 1100 mg of sertraline with 900mg of mirtazapine in one dose?

It is dangerous. Th recommended dose of sertraline is 50mg to 200 mg a day and for mirtazapine 15mg to 45mg a day. It is unwise and potentially unsafe to take more of your medication the recommended by your doctor. The doses you speak of are not necessarily lethal but could make you cry sick or if you have a number of medical preconditions (e.g. heart disease) could be very dangerous.