How to use zsh and bash on iTerm2 at the same time?
If by “all the configurations for bash didn’t work” you mean that customizations made in ~/.bash_profile (or ~/.profile or ~/.login) aren’t taking effect, that’s because bash is being invoked as a non-login shell (it runs ~/.bashrc instead).

To make it a login shell, the simplest solution is to add the -l option:

bash -l


Alternatively, if you want to actually create a login session you can invoke it via the login command. This creates a new login session (as displayed by the who command) and it invokes bash with the name -bash (as displayed by ps) to indicate it’s a login shell (same effect as the -l option):

login -f username bash