Hi Sofy,
IOUT1 and IOUT2 are used when there is a need for an excitation current in the application. According to the datasheet, when the desired output current is 1 mA, you have to configure the IO register's IEXCDIR1 and IEXCDIR0 bits to either 00 or 01 and IEXCEN1 and IEXCEN0 bits to 11.
Since both excitation current are available all the time in this configuration, if IOUT2 is not used in your application, I think it's okay to just leave it NC.
Regards,
RC