Double partners with Apex Clearing who holds your assets in your name. You can create an account through their web portal as an individual investor to view the holdings you have.
First go to Apex's website https://apexfintechsolutions.com/ and login to Apex Clearing.
You will be prompted to sign in or can Create User ID. Please note this is different to any login information used with Double. You will need to create a completely separate login.
Apex will verify your information by asking for some identifying information as well as your Account Number. You can find yours on the Double Settings page.