I have tested the Overlay using several different site collections (different top parent sites) under different managed paths. They are all on the same web application but different site collections – and the overlay worked fine. I don’t think you are correct when you say it only works within site collections. See enclosed blog link for confirmantion. http://blog.furuknap.net/view-multiple-calendars-in-sharepoint-2010
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June 23, 2010 pm30 8:54 pm
Well done. I’ve heard many feature lists from 2010 and hadn’t yet been exposed to this one. Very cool.
July 26, 2010 pm30 1:42 pm
Is is possible or anyother way to overlay calendar across site collection?
August 2, 2010 pm30 8:25 am
Not through this mechanism. You can probably accomplish this using SPD 2010 and making calls to other site collections using web services.
August 27, 2010 pm30 11:25 am
Excellent information Asif. Tried it on mine, didn’t work. Follow KB 982982 (what a number
) and advice from community, still no luck 
August 30, 2010 pm30 6:48 pm
Sorry to hear that this is not working. It’s pretty basic out-of-box functionality. If you do get a resolution to this, post it back here.
December 8, 2010 pm30 1:18 am
Hi, I just tried it and it worked just fine on my box.
August 17, 2011 pm30 11:31 am
Asif,
I have tested the Overlay using several different site collections (different top parent sites) under different managed paths. They are all on the same web application but different site collections – and the overlay worked fine. I don’t think you are correct when you say it only works within site collections. See enclosed blog link for confirmantion.
http://blog.furuknap.net/view-multiple-calendars-in-sharepoint-2010