Nagaro:
- what is the maximum distance that a ir proximity sensor can detect ?
Depending on the actual sensor up to a few meters. The one pictured, 10-20 cm or so, but it depends highly on the object, the incident angle and the level of ambient IR.
- can i mount both ir sensors together to build a break beam sensor (like the picture from dougp) ?
A proximity sensor looks for the presence of an IR signal, a break beam sensor for the absence. If you would place two proximity sensors opposite one another they would not give any signal: when there's no object presence they see the transmitted IR of the opposite emitter, when there's an object they see the reflected IR of their own.
If you want to go the break beam route, get a break beam sensor.
- what is the added value to have a capacitor ?
Depends on where (both from an electrical and physical standpoint) exactly in your circuit you plan to add one, what value, and maybe even what type of capacitor it is.