SpanningSync has posted on their blog about how to use the SpanningSync application to view multiple Google Calendar accounts. Essentially it boils down to sharing calendars from your non-main account with your main account and then rebooting SpanningSync. While it's not as elegant as some people would have hoped, it's very functional and probably applicable to several of the other Google Calendar sync programs that I've written about today.
Also, if you are using an older beta of SpanningSync, they did release 1.0b16 on March 6th with some bug fixes, etc. -- always a good idea to install the newest version when using beta software and to back everything up before installing the new beta. Unlike previous beta versions, if you quit and relaunch the SpanningSync preferance pane, the update should download automatically.
SpanningSync is certified to work with Google Apps Premier.
Link -- SpanningSync blog about multiple Google Calendar accounts
Link -- SpanningSync 1.0b16