[20171020]

I sadly don't have anyone to do this with but I just wanted to say that I love this idea and doing this with the bottles randomised sounds incredibly fun! You won't know who has which bottle until one person starts drinking. And maybe they have the unmodified bottle but they pretend that it's piss so that the other person thinks theirs is unmodified! Or the person with the piss drinks first but tries to hide it so that the other person thinks that they will have to drink piss and puts it off for as long as possible. Plus mixing and sharing your own piss with that of a special friend is hot...

Another option (which is where I thought this was going) would be to have one person piss in one bottle and the other person piss in the other bottle. Then you don't know who's drinking who's piss, unless you're very good at recognising the taste of your own piss. Maybe you will never know who's piss you actually drank, unless you repeat the exercise a few times at which point you know that it's likely that you drank the other person's piss at some point but you still don't know when.