2 lines different lines of text on one sign

Using Arduino LEDs and Multiplexing
4

I don’t know anything about this matrix library, but have you tried just defining a second object just like the first one and using it the same way?

MaxMatrix m(dIn, cs, clk, maxInUse);

Another line like this but with a different name and then use it just the same way. Does that not work?

It would be my first guess.