If you're going with one computer, then you need software to box with.

I'll recommend IS Boxer for this, although there are other options such as HotKeyNet, Keyclone, GameCommanderPro, AutoHotKey, Octopus, etc. There are free options, one-time fee options and subscription options, depending on how powerful/easy-to-setup of a software you want to go with.

You'll need an assist method. IS Boxer does FTL (which is the best system) automatically, but this can be implemented with any system. If your focus is mainly PvE, a target based system is strong. If you want to run PvP, Focus is a decent assist system too, but competitive PvP will want something where Focus is free for focusing your fire.

The basics of boxing is an assist method and follow. You can combine any two classes, via a click castsequence. Addons are up to you, I like a unit frame addon and something like Quest Helper or Carbonite. For boxing specifically, definitely check out Jamba, by Jafula; it makes a lot of boxing much easier.

Take a look at Click to Move and Interact With Target, as they make melee boxing much easier, and even if you are using 2x Ranged/Casters, they are a great option for quest givers and flight paths.

Google, combined with site:dual-boxing.com is probably the best way to find something here, if it is not stickied. If you take the time to look for yourself, the community is helpful on stuff you cannot find.