training archer fish

can't train them, jus dont feed them and sooner or later they'll start spitting.

They don't really need "training". Stick some blood worm on a leaf, and run for cover.

You'll probably have an issue keeping just 2 archers though, they fight aggressively in pairs, and do better in 5's or more, or alone.

My former archers used to spit at algae on the glass tank lids during water changes, when the water level is down.

Lower the water level and stick dying crickets to the glass.

Its an instinct, they know how to do it and will do it when they want/have to. No training involved.

Archers would rather jump for thier food than spit. If they jump to capture their prey it's almost a sure thing they'll eat the prey. If they spit, they odds are likely that another member of the shoal will actually capture/consume the prey. So in order for them to spit, the food item will need to be outside of their jumping range. That's just a feeding fact that may or may not be useful in your quest for spitting.

The second factor to consider is how far they can actually shoot accurately. A full grown adult, at 10-12", is accurate up to 5'. Do the math and scale down, is it possible you're placing the food too far away?

They'll spit sooner or later. Maybe they're not healthy? You can tell a lot about how archers are feeling based on their coloration. I'd also recommend a varied diet of frozen foods, live insects, and carnivore pellets if possible.

Lower the water level in the tank. Stick a brick of dehydrated tubifex worms just out of reach. They will jump for it. Then later you raise the brick a little at a time. Then when they can't jump high enough to reach the food they will shoot water to get it.

make sure they're in brackish (i'mpretty sure you already know because you posted here.) and are in a big enough tank. when i was young i tried keeping it in a 20 and it was way too smalland never shot.
make sure that the lights are covered cause cold water hitting hot lightbulbs is BAAAD!!!
they also don't seem really interested in crickets at all try different foods. and lower the water as much as possible. the jumping thing is true.