So it's about a girl and a boy (maybe brother and sister). Each of them has one half of a ring and when they join those halfs and say some sort of magic words they summon some giant ghost who fights evil.
That's all I can remember :(

Do you know the name of this cartoon?

Sounds like it might be the Wonder Twins - they were on the Super Friends show...

Maybe a genie?

Sounds like

Shazzan

view opening (http://www.youtube.com/watch?v=cAjvel8iHog)