Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
You can pre-load a Microsoft 365 subscription for up to 5 years, if you have codes for two Microsoft 365 subscriptions for the exact same version (Personal or Family), if you add them one at a time to the same Microsoft account online, then that should extend your subscription expiry date up to two years, if you have performed that process and checked the expiry date on your subscription, does it not reflect the correct date, if not, log out of your account and back into your account to test if the correct expiry date is indicated.
If not, was there any error when you added the two separate codes to your account.