I have done this many times. When I buy from DL on 006 ticket stock I get 100% MQMs. When I buy on Westjet ticket stock from Westjet website I get 50% MQMs. With Westjet there is no flexibility with the ticket you need to fly those exact flights or change your ticket with westjet. With DL you can change the DL flight but not the Westjet flight if you had an onward connection within Canada. I always buy whatever is cheapest.