That happened to me as well when I went through High Rank for the first time back on PS4. What I started doing after a while was to back off a little so the two monsters would get into a fight. Works pretty nicely for certain monster combinations (the two Diablos are pretty much set to fight, but there's others as well)
Another (way funnier) method is to stop fighting and positioning yourself in ways so that monsters attack each other by accident. This will deal less damage to them than territorial battles, but you can actually use certain monsters as shields using this method. And monsters like Bazel deal tons of damage either way.