Query

I have heard that you cannot grow tomatos and cucumbers together in the greenhouse - is this true?

Not to my knowledge. Both mine did well in the same greenhouse last year. They probably would have liked to have their own growing conditions slightly tweeked in their favour but they seem to do well enough.

The reason they say not to grow tomatoes and cucumbers in the same house is because tomatoes like low humidity and cucumbers like high humdity.  Having said that, I too have had success growing both in the same house!  

Richard

As long as you don't put them in the same growbag/pot, or grow them close (and I mean like next door to each other). Last year I grew mine on opposite sides of the green house and it was fine. My green house isn't huge either!

i put toms nearest the door & cucumbers at the back

Ditto, had very sucessful crops from both together in the green house

I put my cucumbers in pots each side of my 8x6 greenhouse & tomatoes in growbags along one side - it works well"