You can automatically add your own signature to documents when you send them out to be signed, or you can sign (or mark an approval) on a document-by-document basis.
On the PDF xEdit page add an 'Auto-sign at sending' field to a PDF document. You can upload a signature which will be applied and recorded to the audit log, every time that document is sent to be e-signed.
If you prefer to counter-sign after the first signer, go to the PDF edit and drag and drop in a regular signature and save it as 'Signer 2'. When you come to send the document add your own details as signer 2.
If you just want to approve what the first signer has done, add yourself as an 'approver' for signer 1 when sending out the document. Don't forget you can add fields for an approver too, this can be a good way for you to add more text, or associated files, to the final document before it is fully completed. Learn more about approvers.
Contact support to create the best workflow for your context.
Check out this video that shows where and how to add your signature to documents to get applied upon sending: