They have had two person finals in the past (Ramps/Dawson, Hollins/Whittle and Matt vs Alesha) and they worked, but they were not ideal. And the incident where Ricky W, as one of the two finalists, drove into a photographer in the week preceding the final must have taken years off the producers' lives.
They won't scrap the dance-off this week if they think there's a decent chance Ugo will return. But I assume they're working on contingency plans for next week. The one thing they absolutely can't do is have people paying money to vote in when nobody's going to be eliminated. The voting public need to know the rules in advance.