YYC has always had two flights to LHR, both of which a well booked most of the year. AC and CP both flew this route, AC with a 340 and CP with a 767 (or DC10). When they merged, both flights were kept because a good number of passengers came from YEG and northern Alberta [as well as Saskatchewan]. I suspect the addition of non-stop service from YEG to LHR has diverted some of these loads, and thus resulted in two 767s rather than two 340s. This also allows redeployment of the 340s elsewhere in the system.