Despite mobs being able to control ridden mobs (i.e. a skeleton controlling a spider in a spider jockey) it does not seem that players are able to control ridden mods via /ride. This seems inconsistent both with mobs being able to control mobs, as well as players being able to control normally-rideable mods.

A fix to retain previous functionality could be to have a boolean argument on the /ride command to decide whether the mounter will be able to control the mountee, or a tag for mobs that, when true, means they are controllable by the player riding them.