How do I force applications to use different internet connections?

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http://askubuntu.com – I have two data connections. One is USB Dongle with high speed limited data plan. The other is a broadband (over WiFi) slower but unlimited data plan connection. I want certain applications such as Software Updates to use only the WiFi connection for downloading whereas my browsers (Chromium, Firefox) to use the high speed connection. Is it possible to force this selection on the applications ? Also, in case one of the connections is not available, then everything should fall back on the other connection. (HowTos)