I've never heard that before Hank and have grown them together in the greenhouse for years.

I've never heard of that either, but I did have a lot of trouble with red spider mite on the cues and had to resort to organic "bugs" which feasted on them.

I believe that tomatoes are happier in a drier atmosphere, whereas cucumbers like a more moist one. A more moist atmosphere is supposed to control the spread of spider mites as regular misting seems to help.

I grow mine outdoors, but my MIL grows both together in a small greenhouse without problems. And she's an 'old wife'. :)

Thanks all, they go together as usual

I think the issue was how easily pathogens can be transferred from one plant to another and putting a wide space between plants was for preservation. Some people, like me, don't have a lot of space to work with.

Thanks B, I set my usual 12 toms which showed no signs of growing. Then I set 12 more to replace them and both lots grew ! So I'm crowded myself.

I did that too. Just 2 plants produce more tomatoes than I can possibly eat. My freezer was stocked with frozen tomato sauce and I was feeding the neighborhood. That was last year and I still have tomato seedlings popping up all over the place.