We had an 85 minute connection TG-> PG last week and barely made it because of a 30+ min delay departing SIN combined with routing around a storm and getting a bus gate. Also, while we had bags checked through they couldn't print a PG BP in SIN. We were required to get them at the transfer desk in BKK.

Most of the time 2 hours should be fine but it is slightly risky. Should you miss your flight I'd expect you'd need to buy a new ticket. If you decide to take the 4 hr layover there is a PG lounge you can use.