How can my app make a new twit with subscribed account?



As far as I understood, I should have OAuth token values (access token and access token secret value) for account (let me call this account as X) from my app, although I have once made subscription to the account X at my app.
I will then combine my app’s consumer key & consumer secret with X’s access token and access token secret to POST statuses/update from account X.

However, when I used access token and secret which I received when subscribing X on my app, it does not work correctly (saying that authentification is failed or something). What is wrong with here?

Also, I think usual 3rd party twitter clients which do something like this save OAuth token values in local storage, but my app works on the heroku server (with NodeJS) so it does not have any sustainable storage.
I want to hard-code access token and secret values for account X in my application. Is this possible? (Security issue is of no concern cause X is also my secondary account)