You can create an envelope based on multiple templates. The trick is to use the compositeTemplates optional property of the request.
Back in April 2013 I hosted a webinar focused on templates where I demonstrated 3 different template examples. They were increasing in complexity leading up to the third, where that last one shows how to combine multiple templates into one envelope. Here is the Gist for it, which contains PHP code and sample JSON bodies:
https://github.com/Ergin008/DocuSign-REST-API-Webinar-April2013
Here is the JSON for the third example, you'll want to add something similar to your request body:
{
"emailSubject": "DocuSign Templates Webinar - Example 3",
"emailBlurb": "Example #3 - Composite Templates",
"status": "sent",
"compositeTemplates": [
{
"serverTemplates": [
{
"sequence": "1",
"templateId": "55A80182-2E9F-435D-9B16-FD1E1C0F9D74"
}
],
"inlineTemplates": [
{
"sequence": "1",
"recipients": {
"signers": [
{
"email": "test@docusign.com",
"name": "First Recipient",
"recipientId": "1",
"roleName": "RoleOne"
}
]
}
}
]
},
{
"serverTemplates": [
{
"sequence": "2",
"templateId": "44D9E888-3D86-4186-8EE9-7071BC87A0DA"
}
],
"inlineTemplates": [
{
"sequence": "2",
"recipients": {
"signers": [
{
"email": "test2@docusign.com",
"name": "Recipient 2",
"recipientId": "1",
"roleName": "RoleOne"
}
]
}
}
]
}
]
}
Note that the sequence property of the server template can be used to change the order that the template documents show up in the envelope.