It is in fact possible to receive a double-Tactician benefit either from one or two copies of the card with a few different tricks, such as:
Playing Golem from Alchemy. If it reveals a Tactician and another card that draws you more cards (and you also have spare actions), then you can draw into a second copy of Tactician to play and hopefully still have ...
You do the effects in order:
Discard a card per empty supply pile.
When you get to #4, you find that there's one empty supply pile, so you discard one card. The "Discard N Cards" effect means you discard all N cards all at once, not in sequence.
From the wiki:
If you discard multiple cards at once, you do not need to ...
Enchantress cannot stack; the first action played by an opponent will just give +1 card +1 action.
A quote from the creator/designer of Dominion confirms this:
Throne / Enchantress does not give other players +2 Cards +2 Actions for their first Action played.
The reason it works this way is that Enchantress only does something when you would be ...
Because it is a replacement effect. The first one you play replaced it with +1 card and +1 action. The second one you play would replace that with +1 card and +1 action but that is the same so there really isn't a change.
Each additional one you play replaces the effect of the last one meaning it will always be +1 card and +1 action. This is important in ...