Proper tubing install makes a huge difference also. the Masterflex L/S heads several of us use are fixed occlusion(tension on the rollers that squeeze the tubing against the wall of the pump head).When doing an AWC system you really need one or both of the pump heads to have adjustable occlusion. It helps balance the system.
The L/S heads have a 3% tolerance between heads and this is not different enough to make a difference for most applications. Adjustable occlusion heads would be ideal, as you can get them to within 0.5% tolerance. They just come at a premium price tag and are harder to find. But most all the Cole-Parmer Masterflex heads are interchangeable and stack able. So as suggested you don't even need to use two matching heads on the same motor. If you got one adjustable head it could be stacked on with a non adjustable head and used to match flow rate for example.