Uhh.. I'm not for the "cityhouse built just for clan" idea. There is still a control issue because whoever pays the most gets the most keys, that's not balanced at all.
"Oh man, I just was going to deposit some money, but he did before me, so I only get 7 keys. Grrr."
These types of problems arise. Also, if they leave the clan, they should still be able to give it out to whoever they want because they helped build it. Personally, if you want a cityhouse for your clan, and you want the clan to remian sane over a cityhouse, I suggest 1 person builds the cityhouse.
Also, what about if they both leave the clan... then the cityhouse is abandoned, which means more wasted cityhouses... too much wasted time, gems and gold over this idea, sorry.