How do I put only one SIM card in dual-SIM smartphone to "Airplane mode"?

In Airplane Mode, the device lasts noticably longer. The "Cell standby" shows 76% in battery settings. There are two SIM cards inserted into the device, but now I only use the second. I expect that if I disable the first SIM card, it should consume less power. I don't want to remove the SIM from the phone though, as it's easy to get lost or to forget it when needed. How do I disable (as in "Airplane mode") only one of two SIM cards on Android? If there is no proper way, can it be done by some root trick?