I'm trying to display the numbers 0-99 by multiplexing to two 7 segment displays. One number does not display
Missing pinmode(4,OUTPUT) in your setup routine.

As a general comment, using variable names instead of 'magic numbers' makes your code easier to read - and thus usually easier to debug. for example:

const int leftDisplay = 4;
const int rightDisplay = 5;
const int seg1 = 12;
const int seg2 = 11;
[...]


You can also use HIGH and LOW instead of 1 and 0 for the digitalwrite elements