I have a two views I'd like to place in a way that they are equally spaced horizontally.
I'd like to define thier Center X so that they are the third and two-thirds of the width of the superview but I have found no constraint setting that allows you to do that.
Can you tell me how can this be achieved?

You can use a Center X constraint with the following settings:

For the second view the Multiplier has to be 2/3 of course.
Result:

Select Align Center X from the subView and the Superview (screen of only the last one)

Change the multiplier: 2:3 for the first one, and 4:3 for the second one:

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