It's not "forbidden" but there is a reason behind the advice. It's meant to help you avoid coupling from one trace to another, and therefor limit noise or crosstalk between the traces. This could apply to any time varying signal, be it digital or analog. It could apply to your power traces if the flow of current is changing, or they could be the victim if a nearby signal is coupling noise onto them.
One way to avoid this is to route traces perpendicular to each other on opposite layers, and to minimize parallel run length. The only thing that stops or reduces coupling is distance or isolation.
2 layer boards come with their own set of complications when you start thinking about current return paths. Things get messy pretty quickly. There's no rule that you follow and you'll be OK, you have to look at your individual design and decide if the amount of coupling is too much or not. I suppose you could simulate it but surprisingly simulating 2 layer boards is more complex than 4 :)
Multi layer boards with reference planes between them will isolate your outer signal layers from each other (as well as provide a nice clean return path). Just another reason to consider a 4 layer board instead of a 2.