Hello @Cuttlefish thank you for the tag 
I will share today’s update from the overlapping sits thread as it should help here as well:
If anyone is having any issues applying for a sit that starts the same day as another one ends then please reach out to Membership Services via the help desk as this is allowed and any issues need to be reported so they can be looked into. We are sorry if this has caused anyone any issues, it has been tested as working on the platform so reporting individual issues is helpful. Thank you.