@ls08 you are correct, today there is only the /networks/{id}/wireless/airMarshal endpoint for listing out the most recent scanning results from Air Marshal. I'll check if there's an FR in place (or add one) to get the ability to configure Air Marshal and/or configure those dual-radio APs to dedicated scanning mode and back off.
Note that you don't necessarily have to turn them into dedicated scanners if it acceptable to have Air Marshal run opportunistically once a day without having to disrupt client connectivity. More info in the documentation. https://documentation.meraki.com/MR/Monitoring_and_Reporting/Air_Marshal