Based on the DMG, no matter how the damage is spread, whether it's a single attack or more. Still, I find that a CR2 creature is much more threatening if it deals 20 damage to the hit, than two hits for 10 damage at a time.
Analyzing this over 3 rounds, say at 50% chance to hit, there is 12.5% of the worst case (three times more damage for 60 points of damage) for a single attack, while for two attacks, it's just 1.6% (and 3% for 5 or 6 hits). In other words, the maximum damage per turn in 97% of cases would be 40.
Of course, the average is outweighed by the greater chance of not hitting at all, but it seems that the maximum is more revealing of the difficulty of an encounter.
It also seems that as the RC rises, creatures usually have multiple attacks. Of course, this provides the flexibility to attack more creatures, but it also allows a more even distribution of damage.
Should we consider if the monster inflicts all the damage in one or more attacks when calculating the CR?
I would say that having an attack instead of two equates to about 20 to 50% increase in damage (possibly reduced if the monster is supposed to be met in a group and depending on the shot to be reached and the 39; expected AC); but I did not really try to use that as a rule of thumb.