You can definitely make your local node do that though the security comes from the fact that you can't make the network accept the partition you created unless you own 50% of the market cap from their perspective, not the perspective of your local node.
If you tried to spend from a double spend you forced your local node in to, everyone else sees it as invalid.
How do they tell the difference between my fork and their fork?
Nodes take the first block they receive or which ever block is winning in votes as observed by vote traffic coming off the network compared against the set of stake holders currently in the ledger. Unless the stake holders from their perspective are voting for the new block, they won't evict a block.
It sounds like your describing a system where nodes compute accumulated stake in order to find a "highest branch"; RaiBlocks doesn't operate this way.