If you'd like to have a document with fillable fields to be filled/signed by multiple signers - you will use multiple Roles. You would need to be mindful of Roles at every step of the way: when creating the document with fillable fields either manually in web-application or via API using text-tags or placing fillable-fields via PUT call to update the document - every field needs to be assigned a Role. When you create a document invite, refer to documentation about role-based invite vs. simple invite. Note the "order" parameter, which controls the signing-order - whether all of your signers would receive the invite simultaneously, or one after another.
What is the syntax for including multiple obligatory signers in the envelope in signNow?