Create a custom villager with a trade containing things like

_buy:

{id:minecraft:flint_and_steel,Count:1b,Damage:0s}

,_
_buyB:

{id:minecraft:flint_and_steel,Count:1b,Damage:0s}

,_
_sell:

{id:minecraft:emerald,Count:1b,Damage:0s}

_

(should buy two flint and steel for one emerald)
because flint and steels are not stackable that is what I did.

It would look something like <see attachment 1|2>

The thing is though. whenever you put in ONE flint and steel it will already validate the trade and letting you take an emerald. thus only paying ONE flint and steel instead of two for one emerald!

it does NOT matter whether you put it in the first or second slot.
One flint and steel validates.