CPU power and processing speed in computers can be compared to that of the power of an engine. There is never an endpoint where you'd be 100% satisfied with the number of cores it may have or its processing power rating. There was a point in time where an i5 4.5 GHz was a flagship CPU and the one to have, but that was soon replaced with the i7, and more and more GHz each time. Now, enthusiasts have found a way to literally stack CPU's together to make even more processing power!
The concept of stacking computer chips is nothing new. Even manufacturers have developed random access memory cards to have the ability to be merged to double the capacity, and they only require one card slot on the motherboard.
Furthermore, when it comes to internal and even external storage with solid-state hard drives, you can configure them in a stacked sequence to perform the same function and double up on the storage capacity.
Stacking CPU's, on the other hand, is far more tricky though. At this stage, there are no manufacturers who specifically develop CPU's to be stacked on top of each other. But, there are enthusiasts who literally solder the die's together in order to create a far more powerful CPU from multiple chips. And, on top of that, imagine the amount of cooling your PC will need when you're running stacked CPU's!
Take a look at the video below by the YouTube channel, Tech Quickie, on They're Stacking CPU's Now!