Mining Bitcoin and trying to get if from block rewards, transaction fees is hard if you don't have enough big hashrate and get good reward proportion from a mining pool.

Purchasing Bitcoin is not related to Bitcoin mining and you can buy it with your capital on either centralized exchanges or decentralized exchanges. I don't know what you want to say here but if you can run a Bitcoin full node, and use it to store your bitcoin, and make transactions, it is good for your privacy and security.

Running a Bitcoin full node is different than Bitcoin mining too. You can run a full node while you don't join Bitcoin mining. Bitcoin nodes validate transactions. Bitcoin miners confirm transactions.

Mining Competition: The two instances would compete to solve blocks and obtain rewards, potentially increasing the mining difficulty for both networks.
Distribution of Computing Power: Two separate networks could distribute computing power differently than the current network Bitcoin, affecting centralization or decentralization.
Diversification of transactions: Transactions could be processed on two separate networks, leading to a division of assets and liquidity.

If I understand correctly what you mean by the concept of "two separate instances"

No need to discuss software, both activities are to get bitcoins right? Buying bitcoins only takes a few moments say 15 minutes, it doesn't interfere with mining at all which (I think) is a passive activity.

Or let's say you have a mining farm where you can earn more bitcoins than the average miner. Now you sell bitcoins from mining to buy back those bitcoins, no way.
Did I catch your point about someone running two of the same activities/softwares?