When a mechanic issues a safety certificate, you (the consumer) get two copies. The white/blue copy for the MTO to use at a time of transfer and a yellow copy to keep for your records.
A safety certificate is valid for 36 days, however you can really only use it twice. If you have either copy you can use it to put the vehicle on the road.
If a certain MTO office would not accept the yellow one, just try another branch. I have done this several times and have had no issues.
Good Luck.