Wrap them in a class that encompasses properties of their types.
I'm sure you've already figured this out since you've accepted the answer, but in case it helps you or others that see this question here is a code example:
// this would be your class that can encompass the two objects
public class UsersAndPermissions
{
public List<User> Users { get; set; }
public List<UserPermissions> Permissions { get; set; }
}
In your web api method use the above class like this
var usersAndPerms = new UsersAndPermissions
{
Users = MtFacade.GetUser(query),
Permissions = MtFacade.GetUserPermission(User.Identity.Name)
};
// now your json that you return from the web api has both of the objects data
return usersAndPerms;