Each airline has their own ticket stock. CP starts with 018 while AC starts with 014. The number is used only to identify what airline issued the ticket, or in the case of a travel agent, to which airline the money is sent to. The airline would then distribute it accordingly to the other carriers on the ticket when their coupons were used.

As CP has ticketing agreements with AC/UA/DL, there should not be any reason they cannot issue a ticket on CP designated flights unless of course they want the sale to show up in their airlines annual reports.