3 more questions:
Will I be able to tell one sound pulse from another so this measurement system won't get interrupted by another one?
Different wave lengths or something?
This sound pulse won't be audible by human, right?
What if there will be an, let's say, eraser or anything else small between these 2 sensors, will it still work accurate?
What if there will be a sheet of paper?
That are 5 questions 
- No, on 2nd thought, depends on the sensor your use
- to be investigated, you didn't tell that much about the context of your experiment / application / project
- is an option, ultrasound travels at the same speed AFAIK
- don't know, depends on the size of the transmitter/receiver and the thing in between.
- it will at least dampen the signal strength,
Good questions BTW, you need to build and experiment to get the answers, but you understood the concept.