A request can have an external unique identifier, typically the ConversationID. If you don't specify a ConversationID, include the RequestID. The API processes the calls in the order specified.
C# Example
<!-- Moved Gist inline (mc-doc/f4a993b3b1cc91ef83e6883a718055a8) -->CreateOptions co = new CreateOptions();
co.CallsInConversation = 3;
co.CallsInConversationSpecified = true;
co.ConversationID = "1234567";
co.SequenceCode = 1;
co.SequenceCodeSpecified = true;
co.RequestType = RequestType.Asynchronous;
co.RequestTypeSpecified = true;
CreateResult[] results = integrationFramework.Create(co, new APIObject[] { ts }, out requestID, out status);
SOAP Envelope
<!-- Moved Gist inline (mc-doc/e419cc503145beb3586e350fc205e0ba) --><soap:Body>
<CreateRequest xmlns="http://exacttarget.com/wsdl/partnerAPI">
<Options>
<RequestType>Asynchronous</RequestType>
<ConversationID>1234567</ConversationID>
</Options>
...