Dynamic Block Action Parameter Question

I'm trying to create a dynamic block for a drawing title for standard details.  The premise of the block is that it has two titles, one above another.

I have attached an image of the block in Block Editor.   The bottom one (TITLEMARK) is the main title, and I will put in a "display" parameter to show the second title (TITLEMARK2) if we need it.  I want to make it so that when I stretch the line under TITLEMARK, TITLEMARK2 centers itself over the TITLEMARK no matter the length.  TITLEMARK2 will also have a stretch parameter on it's line as well.  Therefore if that line stretches I still want it centered over TITLEMARK.

Any help would be greatly appreciated.

Thanks