If I have 2 herbalists and use both same turn I can move 4 and stack up both ability and reduce terrain cost by 2?
You mean two Foresters, but yes.
If I have 1 foresters and exaust him, then use 1 herbalists to make foresters ready it's ok to use again foresters and have same result as above?
Is foresters + foresters = foresters + (herbalists ready ability)?
Yes to both, as far as using them as you described is concerned.
Now of course you can also use the Herbalists to heal or gain green mana, which is something you cannot do with the Foresters. However, the Foresters have a higher defense value.