CollectionView rows Side By Side - should have same dynamic height

Question

On the IPad I have two different UICollectionViewCells side by side like two columns. The first one have a width of 30% the second of 70%.

This two cells get dynamic content and have different heights. At the moment I return some static height in func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize { ... }

This height is always too large.

So I want that the rows have just one height and this height is the height of the row with more content.

Is there a method how I can calculate how much height a UICollectionViewCell needs? If not, which possibilities do I have?

I tried to generate the height by content like this

    for item in contentInARow {
        contentHeight += 40
    }

But this 40 is too static because the content could be too big or small.

I'm thankful for some input.