I've created 2 Business Rules. The first shows an error message when the Account Number field equals 123456. The second sets the Account Number field value when the Account Name equals "New Account".
To test the first I'll set the Account Number to 123456...
The Business Rule won.
Next I'll set the Account Name to "New Account"...
The Business Rule won again.
But on the bright side, the 2 can work together. If we change the second rule so that instead of setting the Account Number's field value rather to display an error message, we can see that we get both results.