There's no clear - cut answer. Voldemort is extremely powerful in terms of dark magic, but Harry has the power of love and friendship on his side. If the fanfiction focuses on a battle of wits, Harry could be dominant as he has shown great intelligence in the original series. But if it's a straight - up magic duel, Voldemort's years of experience with dark magic might make him seem more dominant initially. However, as the story progresses, Harry could grow and become more of an equal or even surpass Voldemort in terms of influence within the trapped situation.
It could be either. If Voldemort still has his full power, he might seem dominant at first, but Harry has his own inner strength and courage that could make him hold his ground.
In this fanfiction situation, it really depends on the setting. If they are trapped in a place that favors dark magic, Voldemort might have the upper hand. However, if it's a place that values love and friendship like Hogwarts, Harry would be more likely to be dominant. Also, Harry's ability to think on his feet and his loyalty to his friends could give him an edge over Voldemort, who is often alone and distrustful of others.