Currently, there can only be 1 x signature from the Client & 1 x from the Vendor. There isn't a way to have multiple guest signatures within one Document. If you set up the form with 2 x signature from the Client OR the Vendor, it will not let them sign it at all.

As a workaround, you can create a separate Document by cloning it for the second guest to fill & submit if you require a record of both signatures. This article will step you through how to Clone Documents.