How does your actual samba config look like or don't you have configured anything at all currently?
So for your problem of share b, you can add lines similar to the following ones on a per share basis (in this case your share b):
hosts allow = 192.168.115.0/24 127.0.0.1
hosts deny = 0.0.0.0/0
But to get it right, you want to mix the auth shemes from username + passwd for share a with domain access for share b? If so I am not sure if this is possible as I though this can only be definied for the smb server security modell on a general basis.