hahhaha , this is genius.

Is there a tag for this riding on shoulders thing? Surely there are more pictures of it, but I can't figure out what the tag is (if it does indeed exist)...

And yeah, I love this picture too. :D

It's usually tagged shoulder carry, but I'm not sure whether it applies here or not since the "carrying" party looks like she didn't volunteer for it.

I am gonna tag it sitting on person, though.

Hmm, true, though you could argue that she's still carrying her despite that. But I certainly get your point. Definitely the sitting on person thing.

It is kinda clever, the lower one draws human torso, and girl on top draws someone's head.

Nothing beats "Girl on girl"...
somebody shoot me...

Pixiv:
https://www.pixiv.net/member_illust.php?mode=medium&illust_id=56234448

DeviantArt:
https://www.deviantart.com/tomiokajiro/art/sister-283034475