I have never heard of such a unique idea but they would indeed create a hard fork for both the coins. The very idea of a blockchain is that each block will pave the way to the genesis block. When you have a split in chain just like the BCH hardfork, there would be a split off in blocks as well and both the coins would be following 2 different chains each one different from other right from transaction speeds and total supply. How could we combine 2 different coins which has separate features of its own? By writing up a code through which a newly developed coin will have the features of both bitcoin and ethereum? If we tend to do so, we are creating another alt coin but has never merged the 2 separate blockchains.

The small POW blockchains which you are speaking up are never really needed to this world and literally they would easily suffer the 51% attack quite frequently as they are far less miners contributing to the process! Hashrate is related to the miners progression over time and they have nothing to do with the particular coin blockchain.

For instance hackers can attack Foocoin by launching a 51% attack, since they can easily be destroyed and can never withstand such intense mining conditions or overlapping of chain. Rather there is a possibility of communicating between 2 blockchains by writing up a simple smart contract. Ideas like these are still in the development stage, but might hit the limelight sometime sooner.

Combining of 2 blockchains and creating up a new coin would always create a hard fork and if none of the miners are really willing to mine the coin, there would be absolutely null hashrate and they can be destroyed easily. Either way, thanks for always coming up with innovative thoughts and your topics are good to have a read.