If I am taking mirtazapine at night can I take 5-THP for anxiety during the day?

Do not be alarmed about serotonin syndrome, it is very rare, but you must know about it. I advise you to consult with your Dr.
Interaction checker:
mirtazapine ↔ 5-hydroxytryptophan
Applies to: mirtazapine, 5-HTP (5-hydroxytryptophan)
Talk to your doctor before using mirtazapine together with 5-hydroxytryptophan. Combining these medications can increase the risk of a rare but serious condition called the serotonin syndrome, which may include symptoms such as confusion, hallucination, seizure, extreme changes in blood pressure, increased heart rate, fever, excessive sweating, shivering or shaking, blurred vision, muscle spasm or stiffness, tremor, incoordination, stomach cramp, nausea, vomiting, and diarrhea. Severe cases may result in coma and even death. You should contact your doctor immediately if you experience these symptoms while taking the medications. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.

Best solution does not meet your desire for a prescription, however, if ti worked I'd advise sticking with it rather than experiment.