Up until recently, it was not possible to upload large files asynchronously to 2 or more machines simultaneously. This was due to a problem in the ruby ssh implementation. A patch is available which fixes this.
http://rubyforge.org/tracker/index.php?func=detail&aid=17857&group_id=274&atid=1123
If applying the patch is not an option, you can always change the transfer mode to synchronous.
set :synchronous_connect, true
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