Code:
local _, isRidingGroupMount, hasFreePassengerSlot = GetTargetMountedStateInfo(playerDisplayName)
Returns true / true for isRidingGroupMount / hasFreePassengerSlot when the "playerDisplayName" has ONLY selected a dual rider mount in collections and is NOT mounted.
Will return false / false ONLY if "playerDisplayName" has a regular single seat mount selected in collections.