How to make two flex columns, but second should be overflow-y: auto and be of equal height to the first one

Question

I have a flexbox container and two columns in it

What I want to do is to make second column always equal height of the first one (with overflow-y: auto) and not make container grow.

So basically:

<div style="display: flex; justify-content: space-between;">
    <div>
        Long-long
        Content
    </div>
    <div style="overflow-y: auto;">
        Really long content too,
        but the height is always
        equal to the first column
        with overflow-y: auto
    </div>
</div>