When can two countries gain from trading two goods? economics?

I think the best type of example of two countries trading two goods that benefits both countries could be the interchange between Cuba and Venezuela. Cuba sends medical doctors to Venezuela to treat the poor Venezuelans including restoring eyesight and Venezuela sells cheap oil to Cuba so the peoples of both countries benefit not just the elites such as mainly happens in neoliberal dominated economies. The goods should also be unique to each country between the two countries ie red cedar lumber from British Columbia and consumer goods from China say.