i don't see the point of having small amount on coinbase and a big one on bitcoin core, because if they steal from coinbase it mean that your pc is likely infected, which mean that they will steal your btc from core too...
what i'm saying is that having two hot wallet is only giving more bitcoin to thieves
I didn't get your point. Coinbase is an online wallet, so storing some Bitcoins in it will help you to send Bitcoins everywhere. You obviously won't be able to take Bitcoin core with you all time.
About hacks: Coinbase can be hacked without hacking your PC. However, it is not the case for desktop/phone wallets. Splitting the Bitcoins in two or more wallets are good but storing in a place where you don't have access to private keys is far less secure.
ah yeah if you talk about other device such a smartphone then yes, but in that case i prefer to have bitcoin core(or whatever it is called) for mobile, i can't trust online wallet at all