PDF files overlay

Is this a one-time operation? If not, you might want to to re-think your work flow. This sounds like a job for InDesign, where you would flow and format your text into two columns, having complete control over the file. That said, one work-around would be to place the first PDF into an InDesign file, using the place multipage PDF script (Window> Utilities> Scripts> Applications> Samples> placemultipagepdf.jsx). Make a new InDesign layer and place the second PDF on top, one page at a time (tip, if you display the InDesign pages at a reduced magnification, you can quickly place multiple pages without refreshing the screen, all 100 pages of the PDF will remain loaded into the cursor for placing). If the bottom PDF is not showing through, select the top page and change the transparency effect to multiply, export to a new PDF. Alternately, you can export each PDF as a text file then copy and paste into InDesign. Here is a slightly related discussion with some additional tips:

https://community.adobe.com/t5/indesign/number-all-lines/m-p/10893999?page=1#M173643

Yes, But this Layer is only apply one page.

but i have few pages.

Thank you

Did you try to "import as layer"?

You will find it in the local menu of the Layers navigation pane

I am not sure why you say this is a simple case. This is an advanced and unusual case, certainly not something Acrobat was designed to do easily. If possible, do your mail merge in Word, Acrobat supports that.

You would need Acrobat to be able to do it. However, it might require using a script as the built-in function of adding a background file only allows you to select one page at a time as the background image.