Is it feasible to seamlessly combine the resources of two machines into a single virtual machine?

This is very task dependent, and it also depends on exactly how you define seamlessly.

For certain types of loads this already happens. In fact the video editing task you define can do just that in some cases - the term is a "rendering farm".

More generically, many types of "cloud compute" solve this problem.

(The problem with this answer is how you define seamlessley - there are certainly architectures - mainly software ones - that can oversee and make this kind of operation seamless - but they require moderately complex and task dependent setups. Not all tasks are amenable to this model either - particulary where bandwidth is likely to be an issue or where tasks cant be parallelised.)