1.Clusters are used for High-Availability, or High-Performance computing, not gaming; you just send the data to the clusters CPU's, and they crunch away, thats it. The bottleneck in gaming is usually the GPU or CPU, and many threads is not really an issue (yet).
2. You will most likely have to use a Linux distro (as most super computers do & write your own applications to run of them so strike 2 for gaming!
It does appear that all those building a cluster are at least post graduates or phd's and do it to perform specific tasks.
Have you thought about Building a Folding Farm instead with your 2 PC's? Here's a link I found putting “Folding Farm” into Google.
www.talix.com/Folding/Folding_Farm.html
Good luck!
Here is a little more up to date article on building a Beowulf cluster, if you still chose to go that way.
www.linux.com/community/blogs/133-general-linux/9401
A lot of people are doing this using multiple Raspberry Pie's, but the principle remains the same & should apply to your set up.