Okay, if you suffer from insomnia as a symptom of your depression mirtazepine is the correct choice. Have your sleep improved after starting on mirtazepine?
Concerning Olanzapine, 20 mg is a normal dosage if you have been on it for a couple on months. Normally when a new person starts on Olanzapine he will receive 5 mg a day slowly increasing to dosage to 20 mg. 40 mg is the highest amount advised to take. So it seems like your dosage of Olanzapine is fine. It would suggest that you take the Olanzapine before you have to sleep as well and not during the day.
You should have followed the same method with mirtazepine slowly increasing to dosage. The normal dosage here is 15-45 mg each day divided in two dosages (especially if you are in the high end of the spectrum I listed). However, 20 mg of mirtazepine taken in one dosages before you go to sleep seems like a relatively conservative dosage.
Thus, your dosages seems to be okay. Thus, you should, with time adjust the the dosage and this should help with your tiredness. If you have suffered from insomnia for a long time, it make take month before you start feeling normal again btw. This might also be a cause, however less unlikely unless your insomnia was severe over a long period of time. But if you have been slowly increasing your doses to the doses that you are taking now, you might not yet be used to your dose, despite the fact that you have been taking the medications for two months. I think with time, your tiredness will be more manageable as you get used to the medication and achieve a normal sleeping pattern again. After my problems with insomnia I think it took a month of good sleep (6-8 hours uninterrupted sleep) before I started to feel normal again.
Until then caffeine pills are a possibility, but as I wrote before, due to your problems with psychosis, high amounts of stimulants might not be a good idea (even mild stimulants).