In this case the publisher wants 1 single ad slot that can be filled by any of 2 sizes.
It's inefficient because it results in 2 calls to Sharethrough, while only 1 of them may win.
A better way is to configure a single ad unit with the biggest placement. Then Sharethrough's multiplexing feature will try to fill it with an ad of this size or smaller.
Placeholder to replace with ad script