If you mean at the same time, complicated.

Manually; Open one window to site A, then reduce that window to half the screen. Now open a second window to site B, and do the same thing.

you can look at one of these extensions:

• Fox Splitter: https://addons.mozilla.org/firefox/addon/fox-splitter/
• Split Pannel: https://addons.mozilla.org/firefox/addon/split-pannel

I don't know about Windows 8.1, but if you have a keyboard with a "Windows" key, here's a trick that works on Windows 7.

For the page you want on the left, activate that window, hold down the Windows key and press the left arrow to resize the current window to the left half of the screen.

Open the page you want on the right in a new window. Then with that window active, hold down the Windows key and tap the right arrow to resize it to the right half of the screen.

Does that work in Windows 8.1?

Note: holding down the Windows key and pressing the up arrow will maximize the current window.