Just have multiple instances of the element
<cre:createPayment>
<!--Optional:-->
<remitterBankBIC>?</remitterBankBIC>
<!--Optional:-->
<requestMsg>Message 1</requestMsg>
<requestMsg>Message 2</requestMsg>
</cre:createPayment>
If you have their WSDL file you should be able to add a service reference to your project and point it to the WSDL and Visual Studio will construct all the code you need to call the method so it should generate an enumerable "requestMsg" property of the request object as well as methods that call the service for you and you shouldn't need to worry about the exact XML at all. Google "c# add service reference" for more details.