How should I adjust CR if the monsters I introduce are fighting each other?

Question

I'm planning a long one-shot, the setting of which is a large island with half of it sectioned off in an area where massive wild beasts roam and battle each other, through which the party must trek.

As part of this one-shot, I want to include an encounter where two monsters begin the encounter locked in combat with each other (in this case, a territorial Behir defending its territory from a raging Tyrannosaurus). The party needs to pass the two monstrous creatures in order to reach their destination.

With the monsters engaged with each other, and not necessarily the PC's, how should the CR for such an encounter be calculated? The Behir will most likely win, but even so will also likely take damage during the fight. Since there is no urgency for the party to proceed, other than to get away from the dangerous territorial monsters, they could wait until the end of the fight to confront the victor.

Answer 1

Decrease the difficulty of the encounter, without modifying the CR of the monsters.

The Dungeon Master Guide, at pages 84-85, contains some guidelines for encounters that encompass drawbacks and/or advantages for the parties involved in the fight:

An encounter can be made easier or harder based on the choice of location and the situation.

Increase the difficulty of the encounter by one step (from easy to medium, for example) if the characters have a drawback that their enemies don't. Reduce the difficulty by one step if the characters have a benefit that their enemies don't.

[...]

Situational benefits are similar to drawbacks except that they benefit the characters instead of the enemy.

Among the examples provided in the DMG, one can read

The characters are taking damage every round from some environmental effect or magical source, and the enemy isn't.

Hence, this last bit can be adapted to the dinosaurs fighting each other during the fight: the battle will be easier for the characters, stepping down from hard to medium, for example.

The CR system is not perfect and sometimes could be misleading.

Note that sometimes using CRs for encounters' difficulty does not reflect the actual challenge of the fight: see for example How can I avoid accidentally deadly encounters from monsters that are more deadly than their challenge rating suggests?