With Keyclone, if you go to Set Up and then Maximizer, you can change the resolution of wow within each window.

WoW assumes the window is the full monitor, and you can set whatever resolution you'd like within that window. Do the same for the other. If the aspect ratio's are the same for the two windows, swap speed between the two is much faster. If the sizes are identical, still faster. However, the swap speed will have a definite time between each window. Inner Space has much faster swaps if that is important to you.
Choose an aspect ratio (within maximizer) that you like.
You want to run wow in windowed mode. But you can use UI Scale settings to get a lot more screen real estate.

Keyclone broadcasts everything you type to every client.
You have a Pause Key, which will temporarily turn this off, you can configure the key, but PAUSE/BREAK is the default setting.
You can also configure a Black List or White List.
The white list means only keys on the list will be broadcast (I didn't like this setting).
With a black list, I choose to not pass F1, Tab or Num Lock.

You'll definitely want the addon Jamba.
Everything else is optional, but that is almost required for boxing.
It does so many beneficial things.

The easiest macro system for assist (with Keyclone) is Focus Based.
Have a key (on the same keybind for each toon, it could even be the same keybind as your Keyclone PiP hotkeys).
So when you press Shift F1 (or whatever your hotkey is for PiP switch to Account 1), Shift F1 runs a wow macro (on both accounts) to focus the toon on account 1.
When you go Shift F2 (to PiP swap to account 2's toon), both accounts run the wow macro (keybound to Shift F2) to focus the toon on account 2.
The macro will be something like: /Focus [target=ToonA] or /Focus [target=ToonB].

Then all of your other macro's will be:
/cast [target=focustarget] Lightning Bolt
/cast [target=focustarget, help] Lesser Healing Wave; [target=focustargettarget, help] Lesser Healing Wave

You can run pretty much any two toons you'd like.
Or run three toons or five toons.
Two, three and five is the most common, but people box ten toons or even thirty toons.