Is it easier to get an agent for a screenplay if you already have a book agent?

If you have an agency that's got a media rights arm, then the media person might be able to sell a screenplay, but it's not a guarantee.

Well what's your goal?

Most professional screenwriters do not make money by selling a spec script. Most make money by being hired to write (or rewrite) something that is being developed by a production company. When an agent signs you it's not necessarily because they think they'll sell your script (although of course they'd love to if they can). Specs are most often viewed as writing samples that show what you are capable of.

Movie studio people who accept spec scripts get sued. "You stole from my spec script for your "Star Wares" movie. There were 2472 'the's' and 914 'he's' in my script and you stole those." Maybe you think I am kidding. It gets as stupid as that. Writers are not allowed to send in spec scripts because the Writers Guild doesn't want their members to write for nothing. A writer can pitch (talk about his script), though. So people who can't write but have golden tongues get the work. And most of the movies you see today bear that out. It's a lot zanier out there than you think.
Your best bet is to create a "property," a book or short story or something that maybe movie people will be interested in. One established screenwriter wrote a novel hoping to be able to get a studio to film it and give him the job of writing the screenplay. It worked.