As a sender of a proposal, you do not need to countersign a proposal. Only one signature box can be inserted per proposal, and there is good reason for this.
A little history lesson
The original purpose of countersigning was introduced for paper documents to certify that no party had changed either version of the document.
Along with the evolution of digital documents, this requirement is negated and so only one person's approval is necessary - that of your client.
For more on the legal aspect of digitally signed documents, check out our article here.
I want to add my own signature
If you still wish to add yourself as a signatory, whether because of styling preferences, company rules or any other reason - no worries, you can sort this out by adding yourself as a recipient, just like you add your clients, and sign it the same way that your clients do.
The Multiple signatures feature is available on our Premium and Enterprise plans. You can find more information about it here.