There's an arbitrage opportunity, I .e free money, if the fees don't eat your profits.
Polymarket has Harris at $0.38 to win a dollar. Predictit has Trump at $0.55 to win a dollar. Bet on Harris on polymarket and Trump on predictit. You will have bet $0.93 and no matter who wins, you win a dollar profiting $0.07.
This assumes that there are no outcomes such as no one wins which I guess something crazy could happen. But if something like that does happen, you'll probably have a lot more to worry about than your bets.