In the past, many locations took walk-ups, purely based on the fact that they weren't booked with ADRs and knew there would be room. Most people getting a walk up now either arrive right at opening, or end up at a restaurant that likely had ADRs available if you were to call. The more popular locations actually seem to overbook their ADRs and turn all walk ups away (maybe not always at opening). They expect a certain number of no-shows and have already booked to take this into account. However, if you do cancel that extra or unwanted ADR it will go back into the system for the next person to pick up - I'm not sure what the time frame is for getting it back into the system, perhaps if you cancel only a half hour before your time, it might not open up for someone else. Several times people have reported being turned away as a walk up, yet called dining and got an ADR for within the hour - likely from someone who cancelled.
If you are holding 2 ADRs for the same meal, unless your group splits up, its pretty much going to be impossible for you to make both. If you just don't show up to one, there is no guarantee anyone else will get that table that didn't have an ADR. Most likely it means that instead of someone waiting past their ADR time to be seated, they will get in closer to the ADR time. If you go and cancel that ADR, it goes back into the system and someone can pick it up. When table service dining wasn't so popular and when places weren't booked to the hilt, having double or triple ADRs wasn't such an issue. Almost every restaurant had room for walkups regardless of those with ADRs. For better, or worse, those days are over.