How are multiple arguments stored in a message in Switchyard

How are multiple arguments passed to a service in Switchyard stored in the message? For example, if I define a service contract as "public void service(String param1)" I know that param1 becomes the body of the message. What if the contract is defined as "public void service(String param1, String param2)" How are both arguments stored in the message, and how would I access each of them?

In addition, would it be possible to promote some of the arguments to headers instead of keeping them in the body?