Or they will both drop out. Both of them are deeply unpopular which much of the country, and neither wants to be remembered as a one term president whose vanity resulted in an evil opposition movement coming to power and destroying the country. Also, if they lose and the opposition gets control of Congress, Trump or Biden could end up in jail (and in the case of Trump, die a pauper), so they both have an incentive to drop out and support the member of their party with the best chance of uniting their party, and winning independents and discontented members of the other party. And if one drops out an endorses a successor, the other probably will.
Also, based on polling, there is a real possibility of a third party to win the election (for the first time since 1860). Much of the country detests both MAGA and WOKE, and there is still strong support for a foreign policy that favors America leading the “free world” in opposition to Sino-Russian imperialism.