When I was on PTU for two years, my doctor had me take it three times a day. Reason was that it was short lived in our bodies and that taking it three times a day allowed for a more natural dose to be in our system over a longer period as opposed to all at once for about a four hour effective period.
I think it depends on what y our levels are, how the doctor reviews your tests and your lifestyle wether they give it to you once, twice or three times daily.
Ask your doctor the reason for a twice daily dose and ask if three times a day may suit you better. It never hurts to ask why.