You could have a look at ex-business slimline PCs or laptops if two cases are out of the question. They’ll take up less space overall than the typical PC case and are usually pretty power efficient. The former can also be upgraded if you are careful with what you choose and find the right slimline GPUs to fit in said cases. This is what I used to do for my kids particularly with ex-Dell PCs.
The thing is that you’re not going to get a whole lot for your old parts, certainly not more than the benefit you gain by having a second PC, as by the time you sell it the parts would have depreciated in value quite a lot. But, it’s neat I guess that PCs allow you to do stuff like this even if it would result in a bit of frustration.