I wouldn't recommend learning two languages at a time. In my opinion I think that you should learn as much as you can in the language that you are studying. The after that try moving on to something else. Learning two languages at the same time could potentially slow the learning process.
(Being English your mother tonge…)
It's a matter of time. Being unrelated languages, to master Japanese (~2200 hours) and Thai (~1100 hours) at a time may require about 3300 hours. That is too much, in my opinion. Taking classes, 4 hours/day 5 days/week… Is still more than 3 years. Studying by your own, on free time, it will take toooo much.