Once you've imported your seedphrase and see only one account, you should go to Manage and click New. This action will reveal your second account. This is different from MetaMask where multiple accounts are usually linked to one seedphrase.

It seems Sui wallet handles accounts a bit differently. If your accounts have different private keys, you won't see both accounts just by entering one private key. If you're talking about 'seedphrases', here's what you can do: use the Manage option in the wallet and select 'New' to generate the second account.