1) use uint8_t for the pin variables (that is what digitalWrite expects), or as they are constant better use define
2) make a function to set the values
#define PIN5 5
#define PIN6 6
void setMUX(uint8_t a, uint8_t b)
Better would be to define 4 states (enumeration) the MUX can be in, and derive the (HIGH/LOW) values from that. That would make the MUX interface more abstract and it would hide the details of the implementation. In the end you wrap it in a (small) class of its own.