I grow Egyptian onions and garlic right between the tomato hills with no problems.

I've never done it, mostly since while the timing of growth is ok, I'd be afraid that I wouldn't be able to reduce the water for the onions enough to get good dry-down while keeping the tomatoes moist enough for good production. I guess it'd also depend on what varieties you plan to grow - if I grow Stupice as an early tomato, the onions are still bulbing up as the tomatoes are ripening thus there might be some sun related restrictions.

From what I've read, onions and tomatoes are suppose to be grow well together. I plan on growing some garlic and bunching onions with mine this year.

Here is a link that might be useful: Companion Planting