3mm and 5mm IR LEDs with the round-top have narrow-beam lenses. The best way to think of it is like ice-cream waffle cone. So if the detector (D) is within the zone, everything works fine. When shooting a beam across a track you send it diagonally rather than straight across. So there are many placement options that allow beams to co-exist on parallel tracks or near each other. Two are shown below. A stronger beam, whether it be from a 3mm IR LED driven at higher current or from a 5mm IR LED simply makes a bigger ice-cream cone... crudely shown in green below.
Remember we're talking basic IR occupancy detection. It's like peeling layers of an onion, there are more tricks and techniques as you dig deeper.
I'm simply suggesting that to get started, you don't have to break the bank. If DIY tinkering with electronic modules and components is what makes the hobby fun/interesting to you, then for $5 to $10 you can make a bi-directional IR block occupancy detector with signal crossing flashers.
