crop rotation ? toms and tatties

You can grow Tomatoes and Potatoes in the same bed (as far as I know) but you should not grow either in a bed that has had either for the following 3 years

The two are related, and share the same "bugs", so if you grow either in a plot and then either again a year or two later it is likely to enabled any bugs that got left behind from the first crop to really get cracking

The risk is that second time around the infestation is more serious, and therefore becomes more difficult to eradicate.

Rotation is also, usually, designed to facilitate other things - like when to manure, lime, NOT dig (Brassicas do badly if the soil is loose) and so on.

See also: http://www.allotment-garden.org/vegetable/crop-rotation/index.php