My answers would be boring and short. In either case, I'd recommend going up the righthand side of the shoulder, checking one's mirror periodically, and mostly enjoying the ride and that lovely scenery (and consuming snacks!)

Remarks In the scenario where the truckers are both overtaking a cyclist, put yourself in the driver's seat of the rear truck. What position on the roadway will make the cyclist visible the soonest, on the road in this photo? The one that's furthest to the right, since the second truck's driver can't see directly up the road because of the truck in front of him.

In the scenario where the trucks are going opposite directions and will intercept eachother in the immediate vincinity of the cyclist, I'd advise the cyclist to move to the rightmost edge of the shoulder if he/she recognizes that the situation is developing, and do so well in advance if possible. If there's enough time that the truckers can become confident the cyclist is going to hold that line, they'll undoubtedly breathe a bit easier when they pass, feeling some assurance that the cyclist has a decent safety buffer, even if they can't move across the centerline to provide more room themselves (since they're coming head-on at another truck).

Maybe it sounds strange to worry about easing the anxieties of pro truckers, but they've treated me well Name anyone else who's thoughtful enough to dim their high-beam headlights when overtaking a cyclist with a 0.6W Union generator taillight in the middle of pitch-dark countryside (that was me in my younger days). You'd think they considered me a fellow human or something