I know you aren't supposed to advertise on here, but check out
Adventures In Tandem. My wife and I run it. We do small tours (10 couples max), and only allow tandems. Structurally, we are pretty similar to Backroads, but our tours are longer and at a lower price point (and we don't provide bikes like Backroads does). Besides Santana, we seem to only have one other competitor in the tandem only touring business. We think touring with tandems only, rather than a mix of singles and tandems to be a much better experience.