Yes, using CB Code Field and its Code Validation. This lets you validate a field using your own custom PHP. Example as follows on how to do this.
Code Validation: Enable
Code (PHP):
Code:
if ( '[FIELD_A_NAME]' != 'None' ) {
return true;
}
return ( '[value]' == 'None' );
Ajax Validation: Enable
Additional Fields: Field A
Be sure to replace FIELD_A_NAME with the actual name of Field A field. The above would be applied to Field B and covers case 1. It first checks if Field A is None and if it isn't the validation is skipped and if it is then it also checks if Field B is also set to None .For case 2 you'd need to right a similar validation rule for Field A.