The listing agent represents the seller. He/she does not represent the buyer. So the listing agent CAN say "Give me your highest and best offer."
The listing agent CAN NOT say: "The sellers will accept an offer of $x." (Unless the sellers have specifically authorized the listing agent to reveal that information, which is highly unlikely.)
The reason, again, is that the listing agent is representing the seller. It's his/her job to get the best deal for the seller. His/her fiduciary responsibility is to the seller. That's why the listing agent can't/shouldn't reveal a specific number.
Since you say you're using another agent, you should be working through YOUR agent, not communicating with the listing agent.
Hope that helps.
If not please explain a little more what is going on and who you are in the deal.