I'm pretty sure fixed duels are against the rules but yet , you need to get reported to actually be punished for it. The game doesn't have the possibility to find this kind of duels by itself. 3rd party has to see you both doing it and if he report you both then support will look into your duel log and see the clothes and weapons you were wearing ( they are able to do that ) . Again it's not very punishable, you will mostly get 1 day or 3 depending on how many times you have done it.
Long story short, if they don't see you , go for it
P.s.
I don't promote Fixed duels.