Trouble locking fillable form after first signature, before second signature

I have created a PDF fillable form needing two signatures. The entire form needs to be filled out and signed by Person 1, after which the entire form needs to lock except the second signature form, which will be signed by Person 2 after reviewing the form. I expect this is where I use Prepare Form -> Fields -> Signature 1 -> Properties and select "mark as read only/all fields except." However, every time I select that, choose Signature 2, then click OK, the properties automatically revert back to "nothing happens when signed" before I even leave the Properties screen. Am I missing something obvious? How do I get the properties I want to stick around? Thanks in advance!