You can select multiple MetaMask accounts for your Core account once they are connected. Make sure you create a MetaMask wallet first.
1) Click on the MetaMask icon in your Browser Extensions lists.
2) Click the Profile Icon in the Upper-Right corner of the window that appears.
3) Select the account you want to connect to.
4) If you haven’t previously connected the selected account you will be prompted to Connect that account. You can also Connect the selected account by clicking the Not Connected button.
5) Make sure the additional account is selected and Connected or you may run into issues connecting your additional account.
6) Click on the MetaMask button under Blockchain Account.
7) You will then be prompted to Sign
8) Congratulations 🎉 Your additional account has been added to Core!