Hello. If you for example had Part 1 and played to the end before buying Part 2, then when you purchase Part 2 and start from Part 2 it should start you where things left off in Part 1.
If you didn't get to the end of Part 1 before purchasing Part 2 then my best advice is to play through the whole game but skip the scenes you've already seen with the Ren'Py skip feature.