yes, I did use standby database.

I have a primary DB on box1. 2 standby DB on box2 and 3.

user will connect to either box2 or box3. our plan is if box2
fail, user can reconnect to box3 using the same connection
string (user/pass@dba1) from application.

based on Sam's answer, I have to use parallel to implement it.
what if I do not have parallel server, can I do it?