You need to use load balancing or channel bonding
Load balancing is easier to setup and is more compatible. It works with all network devices, but it doesn't guarantee that speed will double (or a multiple of the number of connections) since packets from a socket to another socket still goes through a single connection. The throughput of the whole system is multiplied, but for a single app then only apps that can take advantage of multiple connections may be faster
There are both hardware load balancers (quite expensive) and software load balancers. Without any 3rd party software Windows 7 and below can already use multiple internet connections with a manual metric settings and some configurations. It isn't real load balancing but it basically works. For more information read
Windows 8 and up can automatically use multiple connections without user concerns. You can also use Connectify Dispatch like Journeyman Geek said (update: it's now discontinued and changed to Speedify which is a channel bonding solution, see below) for simpler config and better results
Alternatively use a Linux virtual machine that's specifically set up for load balancing as a proxy, VPN server or something like that. The most famous one being pfSense
Channel bonding works at a lower level, hence requires setup on both ends. That means it'll be less compatible, but all applications will enjoy the benefit of faster connection
As noted above, instead of Dispatch now you'll use Speedify
Dispatch is no longer being sold as we have transitioned to Speedify (http://speedify.com). Speedify is a completely new product, separate from Dispatch. Whereas Dispatch was a load balancer that could only help with downloads using multiple connections (BitTorrent, or downloads using a download manager), Speedify is connection bonding, with support for combining more types of Internet connections and works for more of the things you do online, including regular file downloads, video streaming, and uploads. Try Speedify for free today!