You have to pass the SSL options as a hash. This should work:
ssl_opts = {
client_cert: File.expand_path(@some_cert),
client_key: File.expand_path(@some_key)
}
@connection = Faraday::Connection.new(url: "some_url", ssl: ssl_opts) do |faraday|
faraday.adapter :excon
end
This gist has some more examples for using SSL with Faraday.