As @jh1 said, most people are doing this with a targeted motion sensor. That’s pretty straightforward. The reason is that you said when they break the beam at the top and when they break the beam at the bottom. You didn’t specifically say that you wanted to know what direction they were going in.
However, given the use case you describe, I’m guessing that you actually did want direction.
1) option one: dual trip beams to determine direction
Determining direction is not really a good match to mesh technologies like zwave and zigbee because the messages can bounce around the mesh for a while and there is no forced sequencing. For example if you have a contact sensor on a door and you open it and then shut it again very quickly, it’s entirely possible that the Hub received the “closed” message before it received the “opened” message. That’s the price you pay for the benefits of mesh, which is very low cost for both devices and energy draw.
That said, there are trip beam devices you can build and then add a SmartThings notification to. But you would need and a pi or something similar to do the processing to figure out if the person was coming or going.
So nothing available off-the-shelf. But you might be able to build one.
There’s an example in the following thread:
2) option two: dual off the shelf devices ( requires event separation of at least one minute)
If the expected time period Between the first event and the second event is at least one minute, you can do that with smartthings using a motion sensor or a pressure Mat. See the following:
But again, in a mesh network a dual device method will only work if there’s at least one full minute between the two devices events. Otherwise you can’t guarantee that they will be processed in the necessary sequence. They might be, but they might not .
option three) toggle instead of trying to determine direction
Another alternative which would be much simpler, and I think it’s well worth considering, is using a motion sensor at both the top and the bottom of the stairs to toggle the lights. So that if they are on they turn off and if they’re off they turn on. That way you don’t have to figure out direction and the stairway lights up Whether you’re going up or down.
option four) use activity to turn the lights on and inactivity to turn the lights off
But I think most people would just put one sensor to cover the entire stairway area, have the lights come on when it is tripped, and have the lights turn themselves off after a fixed period of time of inactivity. That way you only need one device, which saves you some money, and it should work no matter whether you’re going upstairs or downstairs. That is, with smart things you don’t have to have another device to trigger the off. You could have the first device trigger the off after it has been inactive for specified period. 