Will a multi-wan router double my speed ?

Dual-wan (multi-wan) routers, a.k.a. load balancers, allow you to use multople internet connections, and will generally double the overall throughput of your network, however, they will not increase the download speed of a single connection, or a single download.

They are usefull in environments with multiple simultaneous connections, where different transfers can be routed over different external IP addresses, effectively doubling the combined speed of all simultaneous transfers. One notable exception is using P2P software, or other similar programs that open multiple connections for the same download.

The most common load balancing routers support per-destination or per-packet balancing. Per-destination load balancing means the router distributes the packets based on the destination address. Given two paths to the same network, all packets for destination1 on that network go over the first path, all packets for destination2 on that network go over the second path, and so on. This preserves packet order, with potential unequal usage of the links. If one host receives the majority of the traffic all packets use one link, which leaves bandwidth on other links unused.

Per-packet load-balancing means that the router sends one packet for destination1 over the first path, the second packet for (the same) destination1 over the second path, and so on. Per-packet load balancing guarantees equal load across all links. However, there is potential that the packets may arrive out of order at the destination because differential delay may exist within the network.

Some business grade routers may also support unequal cost load-balancing (IGRP and EIGRP routing processes), setting preferred routes, OSPF cost, BGP path selection algorithm, etc.

Note: Reportedly, some older residential dual-wan routers may have buggy implementations, causing some issues with VoIP,VPN and even SSL connections, especially if using per-packet balancing.