How to use nginx directives to set multiple proxy headers in config?

Questions
1

I am trying to inject nginx directives in the config as follows:

nginx_proxy_proxy_set_header=X-Auth-X509-Cert $ssl_client_cert
nginx_proxy_proxy_set_header=X-Auth-X509-Principal $ssl_client_s_dn

I then start Kong by:

kong start --vv -c /path/to/kong.conf

It appears that the second line would override the first when Kong starts up, as only the second line is visible in the debug statements:

[debug] nginx_proxy_proxy_set_header = "X-Auth-X509-Principal $ssl_client_s_dn"

Is there a way to set multiple headers?

I am using Kong 2.1.4 (without a database).