Oh, sorry for misreading then
Playing on one PC is superior as long as the PC is strong enough to handle it (one CPU core per game instance is ideal, but not necessary). It's easier to configure, costs less to operate, and will be more responsive. You also get more use out of the available feature set --
Video FX only works on one PC for example and lets you pick pieces of different windows to see in your main window, or a blank window on a second monitor. If it were me I would prefer one PC using two monitors, over two PCs each powering their own.
In the end it comes down to personal preference. You can still use ISBoxer's broadcasting and other features on multiple PCs.