I might be wrong but my understanding from recent experience and postings here is that the only way to add your name to a waitlist is online, which means you would need the miles in the account. In the past you could add your name to waitlists by calling in, if you had miles tied up in existing award bookings, like CJ describes. As an alternative however, in case the miles you have to cover the difference between the J and F award, are sufficient to for a single F award, you can first waitlist one person online, and then the second. You do not need miles for 2 awards to waitlist.
This again is a YMMV situation (reflecting inconsistency depending on who you speak to). I know of others who have successfully asked to add waitlist to an existing booking as recently as in the past month.
You're right that waitlisting each F saver separately is an alternative, the challenge to this approach is SQ may only clear one but not the other.