Your description is a bit confusing so I will interpret it as: A. Two OSs running on two cpus and mobos in one pc case, B. Attempting to combine two CPUs to run 1 desktop.
A. Two separate OSs (I'll assume both windows) and connected to the same monitor (just need a switch). One is running your MS office, browser, skype and stuff. The other runs games and each is optimized to do it's task. A work OS and a game OS.
B. Yes the clustering I was referring to is Linux but you can do it with Windows as well. The second PC is running but as an Ethernet slave to the primary (node) pc. You (programs that are able to) can pass it computational code. You won't really be harnessing the CPU to run your OS or programs but it can help crunch some numbers if set up correctly. These clusters are interesting and have been very useful in my computational physics, they sound cool, but they are little good for anything else. Honestly most people find it a bit humdrum but there are exceptions that can enjoy seeing hundreds of cores running at 100% calculating the trajectories of dust as it combines into astrological bodies....In numbers, spreadsheets and graphs.
I hope that was able to address your project ideas but I agree with the Dutch poster that it would be better to build one PC with more ram. Window 10 can run on 32bit and it's pretty good. Secondly if it can get a compatible SSD for it. Still this is about modding and creating. Good luck in your designs. Heaven knows I've built some of the most pointless devices but you learn a lot.