Why A/C blowing hot air on the left vent and cold air on the right vent

You have a low refrigerant charge-- refrigerant enters and boils off the passenger air flow fed side of the car evaporate first- then there is nothing left for the driver's side--- recover refrigerant- weigh it- know that you have less than half of required charge, then find and fix leak, and evac and recharge and be cool.....