Agreed. That would be a bad call. In this case, if there are two offers on the table and you are confident in your choice, then there would be no further room for negotiations.

If you have two comparable offers and you want to test the negotiating waters, it would be best to approach that company first (before accepting an offer).

In some cases, however, you may have decided to turn down an offer as it stands, despite a lack of competing offers. In this case, if the employer opens up negotiations, it would be a good time to have the conversation.