Hi there,
As far as I know, you need to sign them both.
As for linking the application, I also read IRCC's website and discovered the same thing. I have seen unpaid representatives not being able to link their applications.
Is it necessary to be an unpaid representative? If you make sure that you will receive all the correspondence via email, why choosing to be an unpaid representative? Plus by not being a representative, you could easily link the application.
Word of advice, is you decide to remain as a representative, do not attempt to link the application. Constantly monitor your email and ECAS. Make sure that your email account is backed up so in case that you loose the access to the email, you could recover your email address.
''Unpaid representatives cannot use online accounts. If you have an unpaid representative, you should remind them to regularly monitor their e-mail and mail to ensure correspondence is received."
Please also, research a bit more, don't just count on what I said.
Also, remember that you use the up to date application form that was released by IRCC in June 2018, and send the application to Sydney Nova Scotia not Mississauga.
Sorry I could not help more!
Good luck with your application.