First, we'll check if Contacts are already under the same Account. Since, Accounts can have the same name, we may have multiple Accounts similarly named.
Go to the Contacts tab.

Click on the Account Name of each contact.

Under the Contacts section, we should see only the contact we clicked.

If all looks correct under the Accounts, next we'll want to distinguish the Account Names, if they are the same.
Click on one of the Account Names

Click Edit

Rename the Account Name to something different

Click Save
Now, we want to combine the two contact under the same Account.
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Go to the Contacts tab

Click the Edit link next to the Contact who's Account we want to change

Click on the lookup icon next to the Account Name

In the Lookup popup, search for the Account Name.

Select the Account

Click Save on the Contact
Now, we can check the Account to make sure both Contacts show up.
Go to the Contacts tab

Click on the Account Name

We should now see both contacts under the Contacts section.