Well, they are generally not the same size, so that doesn't work off the bat. Cresteds are mildly aggressive, at least in their fed responses to moving items... Tokays are, well, tokays. They eat small birds after all. Which is not to say they cannot be trained and handled properly, just I wouldn't trust them with any roommates.

As a general rule mixing of any species in the same enclosure is not a good idea. Some have done it, yes, but it just seems to have more risks than benefits.

Two enclosures is really the way to go if you are housing both.