is there a standard for chaining x-forwarded-for headers?

Yes, there's a standard: Don't use X-Forwarded-For at all.

RFC 7239 defines the Forwarded header, which has rather different semantics from X-Forwarded-For, and new implementations ought to be using it. Unfortunately it suffers from the same problem you have identified with X-Forwarded-For here: it may be defined twice in a request or contain a comma separated list of values. Proxies are also allowed to delete it entirely.

And yes, there's a best practice: Use a different header name internally.

Remember that X-Forwarded-For and its replacement Forwarded contain untrusted input. It is trivial for a client to put whatever they want in such a header. If you really need to know the public IP address of whatever connected to your server, stick it in a different header. For instance, CloudFlare uses CF-Connecting-IP for this purpose. I've also seen Client-IP and X-Real-IP used in nginx (where you can define anything you want). Whatever name is used, your load balancers should be sending the requester's IP address in some header other than X-Forwarded-For or Forwarded.