how could the block chain split if segwit2x support is above 80% ?
will we need to upgrade our client to use it after segwit is activated ? if yes, why so ?
You would need to have a new client which would be synced to the fork blockchain and import your private keys to my knowledge. Or, if we're lucky, devs will just push out updates to wallets which would automatically connect them to the Segwit nodes and you wouldn't have to do anything beyond permit the update.
I don't think bitcoin will split if it happens bitcoin will lose its charm.
There's a time to fork and a time to not fork, this is one of those times and if you think Bitcoin will lose its "charm" because of that you're woefully mistaken.