What is the penalty for working with onother...

We would like to inform you that he shall be punished with imprisonment for a period of not less than one year and not exceeding three years, and a fine of not less than 10,000RQ and not exceeding 50,000 RQ, or one of these two penalties if the worker works for another employer.
According to the Residence of Foreigners in Qatar below article states:

Article 19
1- A foreigner who is authorized to enter or reside for a specific purpose or to work for a specific entity, must not violate this purpose or join another job.
The foreigner has to leave the country immediately after the end of the purpose or work for which he is permitted to enter or reside. He must also leave the country if the sponsorship is canceled for any reason.
The Minister of Interior may approve the transfer of the foreign worker’s sponsorship to another employer in cases where it is estimated that the country's interest requires this approval.