Nokia 101 - Can I keep one of the SIM cards active and deactivate the other?

Yes. Select Menu > Settings > Dual SIM settings > Standby mode and the SIM card you want to keep active. The other SIM card will be disconnected from network.

To set both SIM cards to be connected to the network, select Dual SIM. But while one SIM card is active, for example, making a call, sending or receiving a message, or transferring data, the other is unavailable.