I had the same problem with my daughter.  I don't put two nappies on her.  I just put the next size up.  So, she wears large during the day and at night time wears extra large. I use snugglers.  my sister needed to do the same with her son, and she uses huggies.  so I feel one solution is just to put the next size on.  Also, with girls, if they sleep on their tummy with their knees under them(bum up) the urine tends to stay more to the front, so fitting the nappy higher at the front works too.(I have been doing this lately after I forgot to buy the bigger nappies when I ran out)