What can I do with a second Staff of Herding plan?

I was farming for the plans for the Nightmare staff of herding and got the normal staff of herding plans. Since I already got the normal staff I have no need for the plans. I can't sell it and I can't AH it.

Is there anything to do with it, or should I just drop it to not take up place?

Throw it away - as you've mentioned its bind on account and cannot be sold on the auction house.

The plans for the later difficulties are not farmed, they're purchased from a vendor in Act 4 so to get the recipe for the Nightmarish Staff you need to do the following:

• Start a Nightmare game on Act 4 Quest 4 Part 2 (The Prime Evil - Enter the Crystal Arch)
• Go to Gorell the Quartermaster, near the healer and the followers.
• Buy the Plans: Nightmarish Staff of Herding.
• Go to the Blacksmith and use the plans.
• Craft the Nightmarish Staff of Herding by having 200,000 gold and the Staff of Herding in your inventory.

It is worth noting that the chance that Gorell has the recipe is not 100% - if he doesn't have it in your game simply leave the game and re-create it until he does. You will need to do this for each difficulty, so nightmare for the nightmarish plans, hell for the hellish plans, etc.

Hope it helps

OK, first take the plan/any other account bound item and put it in your stash. Now go make a new character and complete the first quest. Once this is done, you can take any of your unneeded account bound items out of your stash and onto the new character. Once you are done, leave game, and delete that character that is holding the items you no longer need.