Backyard Sharing

At first blush, yard sharing (via Yglesias) seems like a pretty good idea. However, after a moment's consideration, I have the following three implementation questions:

• How are the houses supposed to be designed which share backyard space? Row houses? Multi-family homes, with independent yard access? Massively tiny street blocks?
• How does the tragedy of the commons problem get fixed? In other words, who cuts the grass, and what happens when its the Harrises turn but those freeloading assholes are too lazy?
• What happens to these developments the first time someone's kid (white, blond, female kid, of course) gets abducted by a yard-share neighbor and Nancy Grace goes apeshit on the developers?

(And, yes, to my zero readers, I am fully aware that this list is almost certainly incomplete, and doesn't involve even the most serious problem with this idea that you brilliantly pointed out. And I'm also not against the idea, just saying "how does this get fixed." Don't be an asshat).