Make a field required based on the value entered in another field

Hi there.  I’m working on a process for our SDLC.  I would like to set up a flow where the user can indicate that they did or did not have a meeting in our process.  If they did, I want to set a field that documents the outcome of the meeting to required.  If they did not, I want to set a field that asks them to explain why they skipped the meeting to required. 

This pseudo code captures what I’m trying to do.  I could see tons of uses for this type of functionality. 

IF <had meeting = YES> THEN

   <who was in meeting -- REQUIRED>
   <why no meeting -- OPTIONAL>

ELSE

   <who was in meeting -- OPTIONAL>
   <why no meeting -- REQUIRED>

END IF