Dual sim vs airplane mode

If I turn on airplane mode does that mean that both sim cards disconnect from the towers?

How do I set it to use the cell towers for one sim but not the other?

The point of the exercise is when traveling far from home to get a local sim card for slot 2 and not use international very expensive calling on sim 1.

Airplane mode is meant to turn off ALL radios (phone modems, wireless, gps), so it would shutoff more than you need.

under Settings > Network&Internet > SIM Cards, there is a toggle next to each SIM card. (SIM 1, sIm 2, with phone #s listed)
Toggle off the one you don’t want active.

*note: I only have one SIM card in, but I can toggle that one.