It's basically a solo bike carrier with a longer rail. Definitely not engineered for supporting the weight of a tandem. We have not tried ourselves, but a tandem couple that we met at a ride have one, and they always secure their tandem with many, many straps all around.
We have a Pendle rack with vertical support in the middle and fork mount at the front end. We are very happy with it and never had to use any securing strap.