Lately I found a prototype of a fireplace ad in Googles Rich Media Gallery which includes the tools you need to get the single ads in sync:
https://www.richmediagallery.com/detailPage?id=8135
it took me a while to get it - but basically you need to 'connect' the ads and broadcast messages between them to control the animation start.
check out my (rough) try based on the above prototype - and please don't be afraid of the (design and code) eyesore you will find: it's been just a test!!!
http://dev.vadvance.com/project/demo/fireplaceAd/test/test_index.html
a few things i noticed:
since communication between the ads takes a little while it's not enough to broadcast a simple "go". The trick is to refer to an independent timer, in this case i decided to refer to the users system date/time. I ended up calculating an overall delay for message transfer and processing, added a few ms as a buffer and calculated a start date/time to broadcast. This way i got an 99.9% sync when starting the ads. You might need to modify the delay caluclations according to your environment.
starting the ads synced is only half of the trick. Depending on a bunch of factors (total duration, loop or non-loop, other ads on the page handbreaking the anims,...) you might have to check if the ads are still in snyc every now and then (and maybe bring them back in sync). As my customer cancelled the project all of a sudden i didn't have a chance to investigate any deeper yet.
Hope it helps
Cheers
Gatekeeper74