Singularity target both Yellow Viktor and a non-token unit. Did Yellow Viktor's ability proc?

No, Yellow Viktor's ability does not trigger in this situation.

According to the RiftJudge FAQ (ID 670) regarding damage-based spells like Singularity:

1. Resolution: Damage spells deal damage to all targets during the resolution of the spell.
2. Lethal Damage: Units that take lethal damage are marked as such, but they do not die immediately.
3. Cleanup: Units only die during the cleanup step after the spell has finished resolving.
4. Simultaneous Death: Because both units die during the same cleanup step, they die simultaneously.
5. Trigger Condition: Viktor's ability requires him to be on the board to "see" another unit die. Since he dies at the same time as the other unit, he is no longer on the board when the death event is evaluated, and therefore his ability cannot trigger.

As stated in Rule 376.2.c.2:

"A Game Object will not be able to successfully be able to evaluate its Trigger Condition, however, if it leaves the zone that its Trigger is active from at the same time that its Trigger is satisfied."