Maybe it is possible by creating your NSLayoutConstraint manually. You could make the second attribute be the height of label 1, make the constant zero, and then carefully work out what the multiplier would be to make the distance be what you want based on a multiple of the non-zero label height.
("|" is the real (outer) container)
("|" is Label2HideableMarginContainer)
Disabling the Top, Bottom, Leading そして Trailing constraints. So seek them out, than set Active to NO on them. This will cause the Margin-Container to have a Frame Size of (0,0); because it does have subview(s); but there aren't any (active) layout constraints which anchors those subviews to it.
Maybe a bit complex, but you only have to develop it once. All the logic can be put into a separate place, そして be reused every time you need to hide smg.