Your seed phrase is the private key. You need to find an app that will let you log in with it. I think @Vitor Pamplona is working on it for Amethyst. I hope @greenart7c3 will implement it in Amber.

Voyage is external signer (Amber) only, so you can't log in with seed phrase/nsec. We only generate a seed phrase so new users can directly start to use the app without figuring out what an external signer is. 

 cant use two accounts simultaneously in voyage is it possible? and  I have one doubt how can logout with default which app itself created with seedphrase how relogin into voyage with same old account? 

 > cant use two accounts sulimultaneously...
No, not yet. I will implement multi account support eventually.

> how relogin into voyage with same old account
If you press logout you will be back to the account Voyage created for you. But if you uninstall and then reinstall the app the account would be lost. Unless you wrote down your seed and imported it into an external signer. 

 my request is voyage should not create automatically new account while fresh startup and new install. it should ask import your account or create new one that would-be good and it should've the logout in the default app.
thank you