Eastbound and westbound tracks are developed to incorporate airlines preferential routings and the Jetstream. It has nothing to do with two engines on an aircraft. A great deal, if not the majority of Trans-Atlantic aircraft are only twins. Some aircraft, depending on the company, must get an ETOPS certification before being able to fly on whatever route they want. AN ETOPS route will take you closer to Greenland and Iceland than it will to the Azores.
The great circle route is the shortest distance route, but a 100mph tail or head wind is a pretty important piece of information.
When the Eastbound tracks are designed, minimum time tracks (MTT'S) the preferential routes, forecasted turbulence, and even radar holes are all deciding factors. Quite often when there is a very strong Jetstream the MTT from JFK to EGLL is the same as ORD to EGLL. KLAX to EGLL is almost always over Greenland.
A long story short, the Jetstream mainly decides where you are going to fly for either East or Westbound. The Jetstream changes on a daily basis, so do the routes.
Hope this helps.