The rules for when a single attacking creature is blocked by multiple creatures are that the attacking player decides the damage assignment order and must assign lethal damage to each blocker in order before moving on to the next. My opponent had Torbran out and attacked with the 2/3 double-strike knight. I blocked the knight with a 1/3, a 3/2, and a 2/2. According to magic rules here the opponent should only be able to kill one of the creatures with first strike damage since the torbran replacement effect comes into play after the 2/lethal damage is assigned, but somehow during first strike damage my opponent was able to kill both 2/2's by splitting. According to gatherer:

(10/4/2019)

If damage dealt by a source you control is being divided or assigned among multiple permanents an opponent controls or among an opponent and one or more permanents they control, divide the original amount before adding 2. For example, if you attack with a 5/5 red creature with trample and your opponent blocks with a 2/2 creature, you can assign 2 damage to the blocker and 3 damage to the defending player. These amounts are then modified to 4 and 5, respectively.

thus the attacker cannot assign just 1 damage to the 3/2 in the first strike step — it must assign 2 damage, because Torbran’s effect is not considered when determining lethal damage for assigning purposes. This happened incorrectly on Arena