Why two miter slots?

Made in the USA Custom fits standard 34 x 38 mitre slots; Slot Width Range:

For one thing, two runners are harder to get sliding smoothly, without binding in the miter slots slot with no side-to-side movement

miter slot runners Is a Wooden T--Shaped Adjustable RUNNER or MITRE BAR possible ? Runners for Ryobi

miter slot Introducing the Miter Gauge Jig and Fixture Bar by An extra hole along the top is included