Arghh...oh dear..yes, these things do happen for us all
As Galina said..that would work..just prick them separate. Cosmos don't mind the disturbance, and if you are careful neither the sweetcorn.
I didn't have mixed up, but I did 'cook' some of my chilli and pepper seeds..something wrong with my propagator and its over heating.
Anyhow..most germinated ok....but few lots didn't show a single little seedling. I put my little pots aside to be taken away...and they got left there for loooong while..no care, no water, all discarded..and YEP...few seeds decided to eventually germinate
5 weeks later some of them came through!