If you are sending a document to be signed by multiple signers you will want to define the Role for the various fields. The Role is who is allowed to interact with that field. By default the signers will sign in the order that their email addresses were entered into the To: field. It is important to keep this workflow in mind when setting up the signature fields.
When a signature field is placed you can set the Role for that field (who can click to sign). Right click on the signature field for the 1st signer and select edit. This will open a window where you can define the options for that field. Select the 1st signers email address in the drop down list for the Role: field.
Repeat this step to associate all the other signature fields for the other signers.
Please watch this short tutorial video to see how to author a multi-signer document:
https://www.echosign.adobe.com/en/misc/echosign-how-to-series-sending-to-multiple-signers.html