How cooperative are the kids? I can see the trail-a-bike being a source of frustration for one twin or the other. I guess they can stop pedaling, even though you are still pedaling, but what about when one of them stops? Would it cause an issue for you? For the other twin?
Are they old enough and interested enough to pay attention to being on the bike for the whole ride? That's another concern I'd have about the trail-a-bike. You might not be able to ride as far with them responsible for their own uprightness and holding on. In the trailer they could even fall asleep!
Safari wouldn't load the Xtracycle page, so I didn't see the picture of the family version--but I keep picturing it with twins hanging off the side like panniers!!! lol.
I think whatever you do there's going to be a transition period where your riding changes, until they can be a little more independent. I think the box bike is a big investment, and if you bought one you'd be through with it before you got your money's worth (kids grow!), unless you were planning on more kids. You're kind of stuck in a spot--too big for the trailer, and the available options aren't going to last until they're independent riders. That's going to change how much and how far you ride, for a little while.
I haven't seen the Xtracycle with the family option, but I think that might be the way to go, with the trail-a-bike next.
Have fun shopping!
Karen