I think Spielberg decides well in advance what his next project is going to be, and then informs Williams what the schedule is going to be, and Williams manages. Since he works for so few other filmmakers, it's not hard for him to pencil it in. Goldsmith, by contrast, frequently overreaches, costing him such recent assignments as RONIN and REINDEER GAMES, and forcing him to share scoring duties on STAR TREK: FIRST CONTACT with his son Joel; or in the case of accepting AIR FORCE ONE, realizing that he needed another composer (ultimately Joel McNeely) to fill in the blanks, since the schedule was so outrageous (most or all of you remember they were doing a last-minute replacement of a Randy Newman score).

Back to Spielberg: I think all the CREATIVE decisions on the music for COLOR PURPLE were made between Spielberg and Quincy Jones, but Jones farmed out the music to God knows how many orchestrators -- and, to his great credit, insisted that all of them share his Oscar nomination. That is the largest number of people ever to be mentioned on stage, nominated for a single Oscar, at one time.

In any case, it is well known that Spielberg is a huge admirer of John Williams (not to mention that they are friends by now), so why should he work with another composer together? They work very well together, so well that in some cases Spielberg does not even supervise the scoring, just letting Williams do more or less whatever he wants to.

...just imagine if it were the other way round - if Williams had scored "Poltergeist" and Goldsmith "E.T."