Hi tecnico,
There is a QoS configuration in which you can choose which users to give priority and greater bandwidth, in this link that I send you you can review how it is configured, in general terms you must assign a classification to the network segment of your users from SSID B (In the “” User Groups “option) and later assign a percentage of the total available bandwidth (in the” Group Bandwidth Reservation "option) in this way you can give more capacity to the users of SSID B.