so the matrices don't scroll across each other, they just all show the same letter as each individual matrix scrolls...

I did play around with two matrices and LedControl and had this problem too. From memory (not stating fact) I had the number of devices integar set at 1 instead of 2

LedControl lc=LedControl(12,11,10,1); 12, 11 and 10 Arduino pins 1 number of Max7219.

It maybe worth checking good luck Pedro.