Balancing the candidates is not their only stratagem for controlling a democracy, of course. They also aim to divide the people in a very fundamental way, to divide the people culturally and biologically. That's one of the reasons they favor Third World immigration and multiculturalism. It is far less likely to combine behind a single candidate. They also strive to control the democratic process in depth, not just at the presidential level.
Candidates typically come up through the system over a period of time.
A presidential candidate typically has been a governor or a senator or a vice president before he becomes a presidential candidate. The stringpullers have been able to test him and evaluate him over a period of time, to compromise and corrupt him, to be sure that he will take orders and will not surprise them or embarrass them. But, as I noted, sometimes they outsmart themselves. And so it went, right up to election day. And then we got the sort of result we have now, with the people in both major parties convinced that the other side is trying to steal the election.