Well, you could try to use 'Bonding Box Width Scaled' instead of 'On Empty', but this will of course move the second text dynamically with the width of the first text.
Something else you can do which I have done before, is make a 3rd text object, positioned where you would want the second one to be when the first is empty, use VL to tie together the Text values of Text 2 and 3 so you only have to enter the text in one place, and use visibility parameters in VL to make Text 3 not visible unless Text 1 is Empty, and do the inverse to Text 2, so that it is only visible when Text 1 has text in it. The setup for that would look like this:

#XPression