Posted: 19/01/2014 at 14:34
In an ideal world you would grow them separately as cucumbers prefer a high humidity and tomatoes prefer low humidity. The reason for this is that tomatoes are prone to fungal diseases which are promoted by high humidity and can spread rapidly under such conditions, which is why we try and grow them quite dry and not over-water them. Cucumbers on the other hand are prone to spider-mite which love hot, dry conditions so we try and grow these plants in very humid conditions which spider-mite don't like.
In reality, most of us only have one greenhouse so grow them together and, if unlucky enough to get a disease or pest problem, are unable to control it well to the detriment of one crop or the other. I much prefer tomatoes to cucumbers so when spider-mite descends on my cuc's, they are sacrificed. Unfortunately that happened every year for several years so I have stopped growing them for the last 2 years and am hoping that has cleared-up any over-wintering mites (there are no effective sprays, organic or otherwise to fully control a bad infestation.)
It's not until you get a disease/pest problem that you appreciate the facts about their different growing conditions!