I fixed it in this way.
Solution 1: Install RAID 1 without encrypt your disks
First, I have prepared both the hard drives (SSD in my case) using "GParted Live" (I created a bootable USB with the GParted ISO):
- 512 MB partition, named "/bios/efi", fat32, flagged as "boot, esp";
- 10 GB partition, named "/", ext4, flagged as "raid";
- (Entire remaining space) /home partition, named "/home", ext4, flagged as "raid";
I decided to omit a "/swap" partition because I have 16 GB of RAM installed, and I use SSD drives, and this server will be just a little home NAS, so I don't need high performance.
Then, using the "Custom storage layout" during the Ubuntu Server 20.04 installation:
I checked both disks to be "added as a boot disk";
Using the "Create software RAID (md)" option, I selected the "/" - root - disk one partition, and the "/" partition of disk two, to create a new "md0" volume (always active);
I created another "RAID md" volume, but this time with "/home" partition (always active), selecting both "/home" partitions to create a new "md1" partition;
I selected the "md0" partition (the root partition created before) and choose "Add GPT Partition", and than I selected format as "Ext4" and mount on "/";
Similarly with "md1" partition (the /home partition), I choose "Add GPT Partition", and than I selected format as "Ext4" and mount on "/home";
After that I choose "done" and I completed the installation.
Solution 2: Install RAID 1 with disks encryption
Here I wrote a quick guide about how I solved. The main problem is related to the "/boot" partition, that can't be inside the encrypted partition, because the grub's encryption it isn't supported yet.
I hope that this guide will be useful for other Ubuntu users.