Raid Assistant - setting up Raid One on 2 drives, does it delete the data of both drives?

So, I have a 12TB drive. - This HDD is formatted in APFS (Case Sensitive). - It is a good quality Seagate HDD, less than a month old. - It is almost full with data. - I have just ordered the exact same model. - I want to set them both as Mirrored Raid 1. **Question 1:** Do I need a third drive to backup the contents of the one with the DATA, or does MacOS allow me to keep the data of one of the drives while mirroring? **Question 2:** being this a software RAID, is it possible to do the operation on one machine, to use it afterwards on another machine (both Mac Mini M4)? Or do i need to do it in the Mac where it will be finally used? (I'm still waiting for some time to receive my second Mac Mini and i'd like to speed up the process).