A bit of a guess here (not really used it much)
add the "scroll to" behaviour to both your objects - this should keep the screen centred between them.
setting the scale to the screen height (assuming this is smaller than the screen width) divided by the distance between the two objects should keep them in view.
You might not want to scale if they are too close so add something like if distance is greater than screen height
as you said you will also need to make sure the scale doesn't get too small.