Hi ilmenator, I had the same issue but with a smt32, one LCD works fine, both -> garbage. It appears when suply voltage with the PC through USB cable, and fix it changing a 74HC595 with a 74HCT595 instead. so now it's working fine, but what you wrote ( and what Thorsten wrote ) is very important to me to understand the problem, may be a voltage level problem? in my case it seems i think due to the use of a 74HCT595, I'm not sure
Thanks to Thorsten and you for this contribution
Albert