From a how-to point of view, it's a bad idea to go for the default.aspx, because you'll have to remember to do it each time.  You could put the code into a text/html module.  Or you could get a script injector module, and use that.  Finally, you could get one of the existing analytics script modules (I have one that is free with source) and modify it to use the slightly different analytics code.

I'm not quite sure I follow the request - is it for the asynchronous call, or is it to have multiple ga accounts updated on the one page?

I might have a look at this for my analytics module.  I already have a backlog of feature requests, it might be time to do another version.