Yellowing Saarinen Tulip Chairs  

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I can't give a definitive answer, but I suspect that any process to restore the chairs would either destroy them or be prohibitively labor-intensive and expensive.
Is it sunlight or smoke that has yellowed them? I think it's an embedded chemical thing that causes the color change. There is UV protective film that can be applied to windows, obviously this does no good retroactively...
Everything breaks down in the sun. Even Sunbrella type fabrics are only warranteed for something like 1000 hours of direct sunlight...
Sort of the inverse of your problem... I've been thinking lately about how my yellow Eames shell armchair will eventually be bleached (more than it already is) if it remains where it is, but it looks so lovely under the rays of southern exposure morning sunshine...
Does one cover the windows with tinfoil? I hope not. That's life.
If you exchange/rotate your chairs regularly (as you would turn a rug, or flip/rotate a mattress, or rotate sofa cushions) perhaps they'll even out somewhat over the next thirty years or so...
At least you've got two complete sets of four to play with.

pictures would be helpful
I'm not too familiar with the tulip chairs, but I used to detail cars and boats where it was common to see yellowing of the gel coat on boats. One product to look into is West Marine Rust Stain Remover. You could also try an automotive rubbing compound / polish. You will never be able to fully whiten the chairs without completely restoring them but it should help.

If all else fails...
If all else fails, there is always PlastiKote. (Other brands are available.)