Sorry if this is obvious, but I can not work it out. In the Dragon Flight Zones the mount button always opens the new acrobatic mount.. but I would also like to have one of the second binding open my ground mounts... if this possible?

You need to do this with a rule.

Go into the rules settings, from the drop-down select the appropriate key binding, create a new rule with no conditions and the action as Limit Mounts and the parameter as Type > Ground.