The Night Ferry, especially when worked via the North Kent route, famously had a regular 4-4-0 pilot even when worked by an 8P MN to ensure it got over Sole Street Bank - at least from what I've read. The former SE&CR 4-4-0s were 3P in the BR ratings, of course.
I think both locos would be working most of the time - as
@D6130 says - both crews would know the road and work in tandem. I can imagine that a Scot crew would be less than pleased if they thought the 2P in front only worked the regulator for a few miles up Shap and Beattock in the near 200 miles from Preston to Glasgow
Also, even though locos - at least those fitted with Walscheart's valve gear - rarely 'coasted' 100% (meaning no regulator whatsoever) because the pistons needed some steam to bring in oil for lubrication purposes. I don't know what the 'minimum' regulator was when coasting (5% perhaps?) but I suspect it would not be ideal for any loco to be pushed along for mile after mile with just minimum regulator applied.