Tricky, since the buttons are so close together.
It’s this device, correct?
Distinguishing between the two buttons is going to be challenging, it may be that the entire face plate vibrates a little when you press either button. As @Alwas said, it may just take some trial and error.
The only thing that occurs to me initially would be to use a very thin pressure sensor, typically film with an embedded wire, and then use that with a sensor with dry contacts that will fit so that the sensor can send the notification. I’m sure you could put it on top of the button, but that would ruin aesthetics, and I don’t know mechanically if you can put it behind the button without affecting the operation. But there is a large variety of these available at relatively low prices for robotics hobbyists.
The pressure sensor itself doesn’t have any way of communicating to smartthings, so you have to connect it to another device that does. But people have done it, it’s the same principle as putting a pressure mat under a sofa cushion or mattress or welcome mat.
I’m going to tag a couple of people who have done a number of different creative maker projects and may have some ideas.