hey matts girl
ahh,the age old question,is size important?,the age old answer,depends what you're gonna do with it & where you're gonna put it
over here we call them double ovens.obvious disadvantage is that if it is going to fit in the same space as your old oven neither oven can be as big as the old single oven,so i'd refer you to the age old answer above!
advantages are the smaller oven usually houses the grill(broiler?) so you can use 3 cooking techniques at the same time-stove top,broiler & oven,you can use both ovens at the same time for dishes that require different cooking times/temperatures,you can keep food/plates warm in one oven whilst cooking in the other.
if i've misunderstood what a slide in double door is,forget what i've just said