The breaker manufacturer is not the governing body for electrical issues, it is the NEC. Also the reason you do not want two 12 gauge wire on a 20 amp breaker is that under normal circumstances a 20 amp circuit does no pull over 13 amp draw. If you have two appliance plugged in, under normal load they could both pull 13 amps or putting a load of 26 amps which will trip the breaker. The more a breaker trips the weaker the springs and the easier it trips. The building owner will be running back and forth constantly resetting the breaker. Eventually the weak breaker will not reset and breaker will need to be replaced. Circuit draw as most on this web site determines the size of the wire which determines the size of the breaker. You cannot use two wire of smaller gauge to double the size of the breaker.
In the example of two 10 gauge wires on a 60 amp breaker, as combined two 10 gauge wires are still only capable of carrying 30 amps before they could catch on fire. Essentially two 10 gauge wires are considered single gauge wires, not the same as one stranded 6 or 4 gauge wire.
The electrician was probably an apprentice who worked on someone else’s license which is common on residential electrical, but is supposed to be reviewed by a master electrician who is supposed to sign off on any non masters work. The city inspector should catch this when he re-inspects for the building permit.