My apologies if I am not familiar with the implementation you have applied in your application. However, in regards to keeping track of OAuth2 credentials of a specific account, I would recommend that you store them in local file such as the one described in this
guide. You should then be able to programmatically point to the directory of the configuration file based on which account it belongs to so that your application can then use it before sending the request.
Also,
access tokens need not be stored, since it is
automatically regenerated when using a client library of the AdWords API. I hope this helps and feel free to write back if you require further information.