Generally the "dual" contract is allowed in Bulgaria as far as the the working time per all employment contracts does not exceed 12 hours per day - in example the employee may have one main contract , where he/she is employed on a working day of 8 hours per day, and a second contract, where the working time shall be 4 hours per day.
Source: Payroll Vendor/ https://app.asana.com/0/1201545221709572/1203772512352312/f