Is it really true?

Most agents do realize that you've queried others, and some of those agents may have the full. Usually, though, you'd say "May I take a few days or a week to get back to you since I have other agents reading the full" not two weeks. That's a little long. And be truthful. No making it up that others have the full. And it's common courtesy to tell the ones reading the full that you have an offer of representation.

I jumped at the first agent to offer representation. He was very good but not a good fit for me. So it's also in your best interest, too, to call or email the ones who have the full at the time.

They don't like it since they want your book <grin>, but they know a good book will likely have multiple offers and it's in your best interest to explore all your options. My agent actually told me to sleep on it and get back to her after she made her offer. She knew I had mulitple offers on the table.

I got an offer from Agent A, notified the other four reading my MS and wound up with three offers. I went with Agent B. It took about, 2-3 weeks and Agent A waited.

This is the most important decision you will make in your writing career. Take your time and think about it. You want a good fit, not just an agent.