30 years! 
Trying to understand your code, I think I'm starting to get it. Right off the bet, I found tiny issue: need to initialize second display (lc.shutdown(1,false); etc).
Also why bufferLong is 7? Should it be 14 if using 2 displays? Or it doesn't matter?
What does this this code exactly performs?
pgm_read_byte_near(font5x7 + ((ascii - 0x20) * 8) + a);
I understand you are reading it from prog memory and offsetting by 20 characters to get right ASCII, but not sure what "*8" is for and what "font5x7 +" does? Your adding something to an array name not array's element? :~
Sorry for stupid questions 
Edit: Ok I hooked up my LED matrices, and your code totally works... except it looks like I have mine wired/arranged differently from yours 
Instead of scrolling from one matrix to another, it scrolls as if they arranged one on top of each other (i.e. image is rotated 90 degrees clockwise, so it's not going from right to left, but from bottom to the top on each). I need to figure how to to rotate image 90 degrees CW (i.e. rows become columns, etc.).