As a glazier from the past....the method you are using is correct....2 things.....first the two cuts need to be exactly over each other....and second..... use 99% Alcohol. ....kerosene will also work for the heat source. Well done..
Nice video. Don't take this wrong but your hand pressure is slightly too much force...also use mineral spirits as a lubricant for cutting...or you could use all-in-oil Or transmission fluid..it's nice and thick doesn't run fast.