Apache SSL Proxy can't find client certificate?

You need to configure Apache to use that certificate file as an authentication mechanism to its proxy backend.

Combine the .pem and .key into one file, and point to it with:

SSLProxyMachineCertificateFile /path/to/combined.pem

Imho what he is really asking for, is an Apache that proxies the SSL requests to an endpoint server, presumably an application server, that does SSL client certificate authentication...

The SSLProxyMachineCertificateFile will not help you in this case as this file contains a certificate by which the apache server authenticates itself with the application server... you don't want that: you want the calling client to use his certificate

Presumably it should be possible for Apache to pass the certificate info it obtains on to the app server, e.g. via AJP, but I haven't been able to get this working so far. I will try and update this answer if I get it to work...