Digitally Sign is greyed out after 1 person signs a fillable PDF.

Hi, 

I have a situation where I have a fillable PDF that I need to gather 3 signatures for.  I am using the 'certificates' option to digitally sign the form (vs adding a specific field for each signature).  When I send to person #1 and he returns it, the Digitally Sign option is greyed out.  If I send to the 2nd person first and he returns it, I can see the 'Digitally Sign' option and am allowed to sign.  We want to go in a specific order and unfortunately the #1 person is the one where we can't sign after him.    

The reason I'm using Digitally Sign is because I want the signature and date/timestamp as one piece of information.  Seems like if I add the Signature fields, those two pieces are separate and don't use the computer date/time stamp.  I'm still learning all of this, so I may be missing something, IDK.  Open to suggestions.

I have both of these other people's public key.  Thought that would reduce these issues.  

Using:  

Here's what I see when the #1 person signs and returns to me: 

Vs. when the #2 person signs first: 

Any guidance is appreciated!

Lynn