If I understand correctly, you are just defining few username-passwords in your Web.Config file as a key-value pair.
Later, you pick the username-password's key-value paid to authenticate. Now, your doubt is on how to proceed with multiple username-password in this flow.
If so,
Since, it's a custom thing, you can multiple logics to keep and retrieve it.
1. Keep a single key with all the username, password in it enclosed in brackets. Split it in code behind at the time of retrieval and then compare to see if anything matches.
<add key="username:password" value="{username1:password1},{username2:password2},{username3:password3},{username4:password4}">
2. Keep multiple username-passwords together in your web.config and then retrieve all of them at the time of authentication. In your code behind, get the values of the known keys and store it that can be used later for comparison.
<add key="1:username:password" value="username1:password1">
<add key="2:username:password" value="username2:password2">
<add key="3:username:password" value="username3:password3">
<add key="4:username:password" value="username4:password4">
There can be various more ways, all you need is to stick with any one and just code to get the pairs in code behind. Use the pairs for comparison.
Keep it in an application variable or cache it to not retrieve again and again.