Why in the world would cukes "kill" tomatoes?
They are not in the same family, which in the context of this thread means they don't have the same disease profile.
You can plant cukes and tomatoes in the same bed, just consider amount of sunlight, angle of the sun, etc. Cukes are less cold tolerant than tomatoes, so need to be planted later, yet tomatoes can grow tall faster than cukes, so if they are side-by-side, the tomatoes may shade the cukes. So it's strictly a geometry problem.
