A seller received two offers at about the same time. He wants to counter both. May he do so?

It's not a good idea to do so because the seller potentially could be bound to sell the property to two different purchasers if both were to accept the seller’s counteroffer. The seller could reject both offers and invite the prospects to submit better offers or the seller could make a counteroffer to one prospect. When a party makes an offer or counteroffer, that party gives to the other party the power of acceptance to create a binding contract.