Editing the server URL is not really possible, because internally this is a very complex matter and there are many possibilities for data loss because data has internally to be copied partially (only the unsynced data?) to a new calendar/address book. There are so many cases where this could result in chaos, so we probably will never add this feature. All you can do is create the second account, export unsynced data manually on the device from the first account and import it to the newly created second account. There are helper apps like ical Import/Export afaik.
I have the same behaviour on a Sony Z3 Compact tablet.
As far as I understand it, the change is not saved and therfore davdroid can not do anything at all 🙂
nethertheless I hope that somebody has a hint for my there I have to look to solve this problem.
A different contact app can not solve the problem (like true contacts) because they all use the same apps in the background which manage the contacts. @zoulhh could your solve your issue and if yes how?