I would be willing to bet that you are trying to proxy HTTPS traffic. If you think about how HTTPS traffic works, it's entirely encrypted from client to server. In order to pass SSL HTTP traffic on to another server, you will need to receive the HTTPS on SRV1 and then pass it on the SRV2 as plain old HTTP. If your destination for the procy rule is an SSL VirtualHost, then it will be receiving HTTP traffic when it expects HTTPS.
This previous ServerFault question talks about how to configure this in detail but the general gist is:
...other SSL params...
ProxyPass / http://ip.address.for.srv2/
This can be worked around, have a look at this thread for an example. It's not very pleasant though, and by the sound of your use case probabaly easier to just offload the SSL at SRV1.