Only because you mentioned the bay area (I'm assuming SFBay), at least one of the bike rental places in SF will rent you a Co-Motion by the day if you want to get a feel for it. I haven't actually ridden either brand you mentioned, but my understanding from those who have is that Santana and Co-Motion tandems have somewhat different handling.
You mention a coupled bike, do you have plans to travel with it?
I've taken the "close enough" approach to many of my bicycles, but I tend to buy used, often ride them first, and only put on as many miles in a year as some here do in a month. I'm normally proportioned enough (or put on few enough miles) that I don't need a custom, but I'm a bit suspicious of anyone who says a full frame size difference doesn't matter for 60 mile rides. If it were a rental for just one day then sure, but not for a permanently owned bike. The one exception that comes to mind is if those two makers have different sizing charts such that the two are in fact almost the same.