I think there is no huge problem judging from what you described.
Taking it by steps, Field A is Mandatory, while Field B is Not. Since A is Mandatory, even if B is written down, it must show the Error message. If B is Empty and A is Empty, it will still show the Mandatory error. Lastly if A is written but B is not, the operation will procede. (That is what I think I understood about it, if the logic is different, please reply with something like an example)
If your problem is showing the Mandatory Error, you need to go to the Layout window, click on the Form in the Grid (Basicly on the square that you can move around), on Component Parameters(In the Layout Properties) click on Layout Parameters and set on True the Show Validation Error. If you are on Mobile Project I still haven't figured it out yet >.< Tell me what you think!