You cannot add two buttons to your Call to action block with promotional content. To show a promotional block with two buttons, you can insert another type of block with two buttons. Here's how to customize it:
1. Choose a Features or any other block with two buttons from the Block Library.
2. Delete all block elements except a title, a subtitle, and two buttons.
3. Customize the title with your offer or other details.
Tip: Depending on block design you can replace the background with a relevant image, too.
4. Activate the Block settings menu by hovering over your block and clicking on "gear" icon.
5. Select both Primary and Secondary button checkboxes to show two buttons.
6. Customize the buttons with links to website pages with your offer.
7. Publish the site to make changes live.
To learn more about Call to action block visit this tutorial.