Can i use 18w charger with ipad 6 and air 2 ?

You can safely use your Apple 18W Power Adapter (charger) with any of your iPads and iPhones.

If purchasing a new Power Adaptor, the small difference in cost will provide greater flexibility when used with newer devices. New iPad and iPhone models benefit from USB-PD fast charging support - this being the standard supported by Apple.

If your devices have a Lightning connector, to connect them to the 18W Power Adapter (that has a USB-C connection), you’ll need a Lightning to USB-C cable - such as this:

USB-C to Lightning Cable (1m)

https://store.apple.com/uk/xc/product/MX0K2ZM/A

Neither the 2014 iPad Air 2 nor the 2018 6th gen iPad support quick charging with the Apple 18W power/charging adapter.

Just purchase the 12 W adapter instead.

Best of Luck to You!

It won’t, but you will not see any benefits from it.

Those iPads will just take as much power input at its designed regulated amount.