What Keith said.
With novels and short stories, it's one at a time and you would want to disclose if some other publisher was reading the same material.
With screenplays, it's normal for a script to go wide and have maybe fifty or more companies reading the same script at the same time. It's just the way things are done. So no reason to mention it, and mentioning it might seem like a challenge. If a producer asks me if anyone else is reading the script, I will be honest. But no reason to mention that if you aren't asked.
Recently I had two companies interested in an option/purchase agreement on the same script and I told each about the other... in hopes of starting some sort of bidding war. The bidding war did not happen. Each offered to *match* the other guy's price. So it came down to which would be the best home for my script.
You don't want to do anything that may seem like it's challenging the producer before they've made you an offer on your script. That's burning the bridge in front of you. A million companies will read your script, it means nothing. It's a job application.