how to balance 2 separated internet connections

hi, I own a small Internet Company with few servers. We have 2 separated internet connections, one is hdsl 2mb the other wireless 16mb. I would like to set up my server in a way that :

if the hdsl goes down all the servers will be still up through the wireless connection and viceversa

could you tell me what software or hardware I need to set up such a system

thanks for your time

riccardo

 

 

 

 

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