I believe there is free software that uses the network to cause the spare computer to act like another monitor for the first computer.
But I don't understand how you want to use the dual monitors? You mean like 2 halves of a whole desktop, or do you mean mirroring the same desktop onto both monitors, or what?
I use the built-in windows Remote Desktop to access the desktop from my main computer on a laptop, over an ethernet network connection. Works great for me, in totally different rooms. But other software can make it like you have an extended desktop where the first monitor and spare computer's monitor are like 2 parts of a single desktop.