Having been a player for nearly a year now (I think) I can easily say that the drop rate is high enough that most of the events you get both girls without needing to spend kobans. The nature of RNG is just that sometimes I have a "lucky period" where for 2 or 3 events I get both girls within the first 3 days, sometimes I have bad luck (no girls yet for me either this event), but in the end over the course of many events it does balance out. The lucky people will have unlucky events, and the unlucky people will have lucky events. The drop rate definitely is reasonable enough, even if I wouldn't mind an increased drop rate after a certain number of battles if you didn't get a single event girl yet, but oh well .