Yay!! My tandem rack (x2Cycle) arrived today.
Assembling the bits to the various bikes was pretty easy, although very sweaty in this heat.
My daughter is at grandma's, and my husband is busy tonight, so I cornered a friend to ride her bike and try out the tandem rack.

Granted the friend is not an experienced or accomplished cyclist, can barely stay upright, but this is going to take some practice!! If I can get the family to stick with it, this is going to be absolutely wonderful. I'm a little concerned that they won't be willing to put the time in to learn how to use it. It's just going to take coordination between both riders.

The other tricky bit will be getting the two of them to practice using it. I got extra connector bits so their two bikes can hook together. The possible combinations are (leader & follower): me & husband, me & daughter, or husband & daughter.

I'll keep you posted on how it works out. I think it's going to be wonderful, and would be great for couples with unmatched strength, or if you have a kid who is about to outgrow the trailabike. The only alternative is an actual tandem which is ~$1000 (used).