Yes , its possible to integrate multiple broadband connections into a device , for failsafe backup and load balancing.
There are routers with dual Wan ports such as this :
Amazon.com: TP-LINK TL-ER6120 Gigabit Dual-WAN VPN Router, 2 WAN ports, 2 LAN ports, 1 DMZ port, Ipsec PPTP L2TP VPN, Load Balance: Computers & Accessories
However , its going to need quite a bit of setup.
Also , for things such as business solution there are lease lines which are damn expensive but highly reliable, T1 lines.
And as Anirban mentioned , you should go for a dedicated powerbackup solution for power outages.
PS: IF you are looking forward to network 50-100 wireless hosts at one moment , you will definitely need a business grade solution for reliability. Its not just about the router , but also upto the utilization of channels and density.
In my recommendation you will need atleast two to three wireless points/routers for this solution.
Looping [MENTION=126812]whitestar_999[/MENTION]
, he knows it better.
