Synchronized capture for multiple USB cameras

I need to capture images from 8 USB cameras at the same time. I don't need to continuously acquire images (just one set every few seconds), but I need the 8 cameras to capture at very close to the same exact time. Just testing with two cameras at the moment, and one camera always seems to capture about 0.4 seconds after the other. Attached is the current VI I'm testing with.
Any idea how to speed this up? I thought having both grab.vi's in a separate frame would help make them occur at the same time, but it's still a lot slower than I hoped for. Thanks for any suggestions.
Attachments:
Grab USB frame manual.vi ‏46 KB

All depends from precision. 100 ms is not bad result. For better precision I would like to suggest to use cams with external triggers. The cameras with USB interface are also available, for example:
http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=2916
http://www.edmundoptics.com/products/displayproduct.cfm?productid=3356
http://www.ptgrey.com/news/pressreleases/details.asp?articleID=230
Otherwise they may be incompatible with IMAQdx (the only DirectShow compliant cameras are supported). Point Gray Chameleon seems to be DirectShow compatible.

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USB + Joystick integration 2.vi ‏57 KB
fafafafafafafafaf.vi ‏43 KB

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Testing Camera 1.vi ‏59 KB
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Synchronized capture for multiple USB cameras

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