1. I read that never store all your coins in one wallet. So wts the solution? should I install two electrum standalone wallets and store my coins in both of them?
One wallet is okay if you are using hardware wallet like Ledger Nano or Trezor. If you want to install two electrum wallet, learn multiSig wallet. That will help you more.
2. I have written seed on a paper to recover my BTC in case my computer got corrupt. What thing apart from seed should I backup to recover my wallet in case my system is corrupted.
Seed is alone enough to recover but you can also get the private key of the address where you deposited the BTC.
3. Strong password is good enough to secure a wallet or I have to do something else also.
If anyone has your seed, they can get all your BTC.