Polytunnel crop rotation

Hi
I am currently operation a 2 year crop rotation in my 18 foot polytunnel, one for each side. Cucurbits (cucumbers, courgette/squash) on one side and Tomatoes on the other.
I swap them each year (I mix salad leaves and other bits in when there is space).
I add manure and seaweed (both of which I have good supply of) each year.

My question is whether or not I am playing with fire? or is a 2 year rotation enough when I am adding the nutrients each year? I have been doing it for 3 years (starting my fourth this season) without problems.

Quite frankly I could fill the tunnel with tomatoes (love them) but there would be no rotation in that   

Bill