I've met several couples (and some singles) on SH, and most of those I've met have had success meeting other couples (and singles), so it's hardly rare for it to happen here on SH. There are two notable difficulties that have perhaps not been mentioned and there's one very useful feature that has not been brought up that can help tremendously.
The first difficulty is statistical; namely that the number of people curious about the Lifestyle always exceeds the number of people actually willing to take the plunge. So invariably, a large number of correspondences or lengthy chatting sessions, will lead nowhere because so many couple - even some which are genuinely interested - are not quite ready to pull the trigger (and some people never work up the courage to agree to meet or else agree but then get cold feet and back out).
The second difficulty is that couples seeking couples are in fact seeking something that's nearly as hard to find as those precious 'unicorns'. For couples to hit if off, either in correspondence or in person, they need to feel that 4-way chemistry, where all 4 people find each other suitably attractive and interesting. Given that we're not talking about playing bridge here, that's a pretty tall order. Very often one couple is into it and the other not sure, or half of one couple (just one person out of the four) is not quite on board, and so they hem and haw and the correspondences and chatting and photo exchanges get drawn out needlessly long. This is why, despite so many couples saying they are looking for a couple, many of them have much greater luck meeting just one person for a 3way. It's not nearly as difficult to find a 3rd, but couples are wise not to advertise the fact that they are looking for a third, if looking for a male, since they will be inundated with messages, winks, album invites, etc.
Fortunately, there is a feature here on SH which can help (though it's not a universal solution), and that is the 'vouch' function. Look at a person's profile and see if he, she, or they have any vouches from others on the site. People who have vouches are people who, typically, have at least gone to meet others (or played with others), so you know they are not likely to be fake accounts, posers, or people who are so nervous they can't summon the nerve to meet. If a profile has been active here for years and they have no vouches yet, chances are they are not the ones who will actually get around to meeting you, but are instead not quite there and getting vicarious erotic fun from being on the periphery of the lifestyle.
So, my advice to all those who are finding it hard to find people who actually want to meet for a drink or a coffee, and are instead getting frustrated by endless correspondences, is just look to see if a profile has vouches. If someone is vouched for by others, then you are pretty much assured that they are not game-players, not posers, not unprepared to meet yet, but actually will agree to a meet and greet after some reasonable correspondence.
Cheers.