Does a second account cost on the same switch?

Hi, I created a second Acc on my Switch to be able to play Fortnite on it. So far it works, but are there any costs? So, do I have to pay a total of 7.98 euro for both, or does it stay at 3.99 euro?

As far as I know, a Nintendo Switch Online membership is only valid for one account.

However, most of the free titles (including Fortnite) do not require a subscription to be playable online.