Currently you have to scroll all the way down and select parent site or meta to switch between the two sites.
I think the best UI would be a tab style interface on the top left. If you look at the beta of WebApps, you will notice that there is unused space on the top left which has a blue background. If there were two large tabs there: "WebApps" and "Meta", it would make switching between the sites much easier.
Right now I use two different bookmarks for both sites since switching between the two isn't very easy. Additionally, you could have little indicators next to each one (similar to the mail icon currently in use) so that you know if you have any notifications waiting for you in the other site, without having to visit it explicitly.