You can access Covidence using your Cochrane Account. This will establish an account under the email you use for Cochrane; thus if you previously created a Covidence account using that email, logging in with Cochrane will lead you to the same account.
There is not currently a way to link multiple accounts, such as a private email with a Cochrane Account log-in.
To log in with Cochrane, click the purple "Sign in with Cochrane" button. Enter your email and Cochrane password. Accept the permissions, and you'll be ready to go!