Sometimes it is needed to have multiple internet connections in one Linux box. To distinguish it from a more common scenario - it means that multiple default routes are specified.
Multiple default routes is a bit of a dilemma - default route is loosely defined as 'if everything else fails send packets there'. By definition there can not be two or more last resort options.
One of those internet connections therefore should be designed as default, and configured normally, giving it IP address, default gateway and so on.
The rest should have its IP addresses configured.