1x 20l Topf oder 2x 9l Töpfe

Oh, that's a tough one! On the one hand, if you have 2 plants and something goes wrong, one plant might survive and the other might fail - but you'd still have one to give you a harvest (all seeds/plants may react differently to the same set of circumstances). On the other hand, a plant grown in a larger container should give you a larger harvest and you would be able to give it 100% of your attention and not worrying about having to do something different to a sister plant... I think, if this was a decision I had to make, I'd go with the single plant... larger yield, more space for her to grow, not having to compete with another for light, better airflow... Good luck with making your decision!