A Power User Subscription would still be required for the Power User features - including custom trading styles, supplementary pages, and so on.
Affinity has different features, which include managing accounts with one email address and password, Companies House tracking for linked accounts, volume discounts and more.
Affinity does not give you the Power User features, and likewise, Power User doesn’t give you the Affinity features. What Affinity does, is helps make managing multiple QuickFile accounts easier, which is why it’s primarily used by accountants and bookkeepers.
The features outlined on the comparison page are separate for each product.
Where Affinity has a benefit is if you have lots of accounts, or where you don’t need to Power User features, but are required to have it because of your account size. As you rightfully point out, it’s cheaper over a year to have 2 L profiles attached on Affinity, than it is to buy a subscription for them individually.
I hope this helps.