Is it possible to use a different set of certs for the client side and another set for the upstream server side?

My use case is to have different sets of local ssl certs on Nginx. A key/cert pair for communicating with clients and another set for communicating with the upstream proxy.

Right now I can define a server module with ssl and specify the ssl certificates and specify a https protocol for proxy_pass for a location. But both client and upstream connections end up using the same certificates specified with $ssl_certificate. How can I specify different certificates for the client side connection and upstream side connection?