It might be simpler to leave the LCD shield alone and use another pin on the other shield.
HI Nick, thanks for your answer! But I'm a little bit eager so I've already made what I ask for ahah, everything works fine, no smoke so far. Now I use the pin 12 as the enable of the shield and change the definition of the lcd in the code.
I think in principle it is best to leave the store-bought stuff alone an make the modifications on your own stuff but, if it works, it works!
Having said that, I submit that pin 12 is a poor choice you may regret later. This is typically used for MISO on the SPI bus, and best left for that purpose. Pins 2-10 are usually all PWM and equally convenient.
I got real value making up a colourful Excel spreadsheet of the pin allocations, enabling me to (hopefully) plan for everything. This particularly applies if you home-brew proto shields!