In case you didn't get an answer yet, I just had the same problem today and I figured it out with the help of a Lacerte/Intuit rep. You need to make two PDF attachments, which are both a copy of the signed 8283 form. On screen 4.4, answer the question about "Link to form" to say it is for the Form 8283 Part B. On one PDF attachment, you say that it is for the Donee signature, and on the other  PDF attachment, you say it is for the Appraiser signature. Also, underneath that Link to form line, you need to enter the Activity name. That seemed to do the trick - I haven't e-filed yet, but the diagnostics went away.