I have a business idea that involves two companies that are well
established. The idea is to put them together to offer a service that
would be profitable for both of them. Of course I would like to
present the idea to them.
A good example would be the promotion that Burger King and AOL have,
that you get a code in every burger wrap to download a song. They
promote each other and gain more mutual customers.
If I were the one who thought of that:
How could I present the idea to these two companies and take credit for it? 
Of course my biggest fear is that they would go for it and leave me outside. 
Is there a way to patent or copyright a business idea?
In case you need to know what kind of business I?m talking about, is
radio broadcasting.

Sincerely,
Mariano Katz

Contact someone at each company, tell them you have a way for them to
generate revenue that they have not thought of, and ask them to sign a
simple short agreement that you'll get X% of the net revenue if they
accept your idea,  They may be uninterested, but you never know.

You can't legally copyright an idea. You can copyright the expression
of an idea that is stored in a fixed form. By US Copyright law (IANAL,
btw, and below disclaimer still applies), copyright occurs immediately
upon the expression being stored in a fixed form. You don't have to
register to get a copyright, but you may have to register your
copyright to obtain legal rights under copyright law. But you are
allowed to register retroactive to the lawsuit as long as you can
prove the expression of the idea was stored in fixed form before the
violation occurred.

Your best bet (legally) would be to get a lawyer to draw up a
nondisclosure agreement that the parties must sign prior to the idea
being presented. OTOH, if you actually want to get the business, I'd
suggest making sure that your part adds more value than simple
matchmaking, unless of course that *is* your line of business.