I'm afraid that's not how that works. If the queue is remaining stable at 20 minutes with one train then putting a second train on will take that queue down to nothing in 20minutes.

Let's use corkscrew at Cedar Point as a random example (because I have the Rcdb page for it open right now)

Thats listed as a capacity of 1800 per hour. So with one train instead of two running that's 900 per hour.

So in 20mins (1/3 of an hour) the ride can shift 300 people.

So if the queue is at 20mins then we can say that there are 300 in the queue and if its staying stable at 20mind then there must be 300 people people joining the queue every 20minutes to replace thea 300 people riding every 20minutes

When you put a second train on then the capacity will double so the ride will now take 600 people every 20minutes.

With only 300 people joining the queue every 20 minutes it will only take 20 minutes after they add the train for the queue to entirely dissapear (its now taken the 300 in the queue and the 300 that joined in that time) and then they are left in the situation where they have 2 trains on the track for no reason.

So It only makes sense to put a second train on if the number of people joining the queue is higher than the number of people the ride can take in the same period. And therefore the queue time is increasing over time. I assume there will be some threshold number for any given ride which means they will add a second train.

Hope that makes sense

(before any geeks point out that adding a seconds train doesn't exactly double capacity and that the time taken to add the second train will add more people to the queue as no ones riding, I know

this is an idealised example)