It is not currently possible to simultaneously manage multiple Stellar accounts/wallets in one LOBSTR account.
If you wish to use multiple Stellar wallets with LOBSTR and further switch between them, you can choose between these 2 options:
Option 1. Create a new LOBSTR account using another email address.
Once the LOBSTR account is created, you'll be offered to generate a new Stellar wallet or connect an existing one to your LOBSTR account.
A detailed guide on opening an account in LOBSTR can be found here.
Option 2. Remove the current Stellar wallet from your LOBSTR account under the 'Wallet' tab in the Settings. Create a new Stellar wallet or link an existing wallet to your LOBSTR account then.
Make sure you backed up the public and secret keys of your Stellar account or have access to the Recovery Phrase before removing the wallet.