How do I put only one SIM card in dual-sim smartphone to "Airplane mode"? In Airplane Mode the device last noticably longer. The "Cell standby" shows 76% in battery settings.

There are two SIM cards inserted into the device, but now I use only the second. I expect that if I disable the first SIM card, it should consume less power. I don't want to put it away from the slow although, as it's easy to lost or forget about 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?
You missed the very first configuration menu item: "SIM card management".

There are checkboxes to enable first and second SIM cards, as well as to choose which card is used for data and for calls/messages.