Yeah, there is no expression to get a sprite scale, while you can set it (as far as I know, I might be wrong, but I think it's because you can modify height and width separately, so there is not one unique scale). So one way to do it is to store the original width or height of the first sprite in an instance variable when its scale is still 1:1 (like on layout start). Then you can calculate the sprite scale at any time by dividing the current width by the original width.
It would be something like this:
Every Tick -> Sprite2 scale = Sprite1.Width/Sprite1.originalWidth
Not sure if it's the most efficient way, but that should work. There might be a workaround that does not require using variables.