Uruk Chieftain from Harad set says: "Forced: At the end of the encounter phase, Uruk Chieftain engages the player engaged with the most non-unique Orc enemies."

I'm playing two-handed, and neither "player" has any engaged Orc enemies. Does Uruk Chieftain not engage either "player"? Or is it a tie of 0, so I pick who he engages?

from the FAQ:: (1.03) Conflicting Effect Targeting
If an encounter or quest effect attempts to target a single player or card, and there are multiple eligible targets, the first player selects the target of the effect from among the eligible options. (The example they give is pretty explicit too, and makes it clear this applies.)

So he does engage someone and the first player chooses who.