When both persons speaking are visible I memorize the lines while the actors practice (having a script at hand might be helpful) and pan the mic between both.
If the scene is split into shot/ reverse shot, I also try to get both but my priority lies on the person on-screen. Especialy when the boom operator isn't experienced, too much panning might overburden him/her with the result that neither actors lines are clear.
Sometimes, I ask the actor if he needs the lines delivered by the actor off-screen for his performance. If not, I even ask the off-screen actor to remain silent so I can also capture little details of the actor filmed. This however can also be counterproductive as it might worsen the performance.
You can also use a second boom/stand or lav mic for the off-screen actor.
If there still are problems (especially if they talk simultaneously), I like to do wildtracks on set hoping to avoid (too much) ADR.
As mentioned in your other thread, production sound isn't my expertise though.