NOTE: Updated 6/8 - still unsupported
This is not an official feature (to be clear - this isn't an officially supported script/workaround - but is provided here as a convenience for users wanting to try this custom script out to add the functionality), but shows a way where you can have multiple add-to-cart buttons for different products or variants. To download the template which contains the script, go to:
Precheckout Upsell.zip - NOTE - this template download does NOT have the updated script below - be sure to update the script manually.
You will need to unzip and then import the .json file as a page.
The script itself is very powerful and has lots of different options. However, in order to make it work you will need to follow the instructions in the video and within the script itself.
Here is the raw script: