Nokia C2-03 - Why the signal bar of one SIM card on the screen always disappears when I make a call with the other SIM card?

Both of your SIM cards can be available at the same time when the device is not being used, but while one SIM card is active, for example, making a call, sending or receiving a message, or transferring data, the other is unavailable. That's why the signal bar of the other SIM card disappears.