To expand upon this, if you're with BT (or any subsidiary or reseller of their services) and you have 2 routers configured with the same broadband information, whilst both will 'sync' to the broadband, only the first to connect will be able to pass traffic.
(found this out when we ordered Infinity for one customer, replaced it with a better router than the woeful business hub which we left with the customer, and the customer promptly sold the business hub on eBay
)