A garnishee order in BC has to be sent to your employers payroll within 7 days of the cutoff period for your pay. So if a creditor knows where to send the garnishee order for both payrolls (if you have two jobs) and gets the garnishee to your payroll within the time period ordered by the court, then yes you could get a wage garnishee at two jobs. They have to file a new garnishee for every pay they want to garnishee, it can be very expensive.
Google search BC courts for the government courts site, they have a lot of information to
stop a wage garnishment for non-legal experts to understand the system.
As far as getting fired, it is highly unlikely. But that's more of a BC labor standards question I believe, check their site.