Which approach of distributing teams makes a successful project: train a singular subject expert or a multiple resources with respect to time?

Training and distribute the knowldge betewwn multiple resouces is great and will decrease the time later.

As you will plan for testing without restrication and allow the team leader with many options during estimate the required time and shall decrease on one resource in the knowledge.

on the other hand , if the project duration is tighted and very crashed and testers are aligned with their tasks without any permissions or delays , you should depend on singular subject expert as you do not have the training time.

Conceptually, training multiple resources (if doable) is much better. That needs to be done with a positive ROU,  within an approved budget, and without impacting the operations plan negatively.

ALL of us are smarter than any one of us. Therefore, I would go for train muliple resources options and save time :)