 As long as the memory is otherwise compatible, it won't be an issue.The bigger issue is that mismatched modules will mean only half the RAM (8G) will operate in dual channel mode, at full bandwidth.  The other 4G will run at half bandwidth.You need 2x4 or 2X8 for full dual channel operation with two modules.  
I want to upgrade my Dell Inspiron 15 5559 (I5 - 6th) from 8GB of RAM to 12GB, but I'm not sure if I can have one 4GB DDR3L 1600MHz 1Rx8 RAM (which is the original one on the PC) and another one with the same characteristics except that it's 2Rx8. Is this possible or could I have problems?
Can I mix a 1Rx8 RAM with a 2Rx8 one?