Where to store second pair of credentials

Although, I’ve personally solved this problem already, I’m wondering how are other people doing it.
Let me explain my use case:

I need to consume a REST API with OAuth. Surely enough, I can store those oauth credentials in the credentials section. Now here’s the kick: to reach that API, the request has to go through a proxy, and that proxy requires a separate authentication step by using a Proxy-Authorization header.

Now, as far I as know, the HTTP node, can import the oauth credentials, which is great, but in my scenario it also needs to import the Proxy’s credentials, since two servers will retrieve credentials from the same request.

In a situation like this (and I have quite a few in my workflows), where do you feel is the most appropriated place to store the extra set of credentials, so that it can be consumed?