install ubuntu without grub2

I decide to install ubuntu disco. Is it possible to do it without grub?

sudo lsblk -f
[sudo] Passwort für alex: 
NAME   FSTYPE LABEL UUID                                 MOUNTPOINT
sda                                                      
├─sda1 ext4         88411b09-1aaa-4775-9fc7-f60041049d6c #debian main system 
└─sda2 swap         c3c5da03-36db-4ad5-8817-05215fe43b6d # woulde be removed
sdb                                                      
├─sdb1 ext4         0ddc8e0a-4b8a-479c-9502-5478ed3c4036 / #ubuntu 18.04 
├─sdb2 swap         927f38a0-c962-47de-9361-f1730032704e [SWAP]
└─sdb3 ext4         1ebf0f02-cdd8-44d9-80f9-7078de79e191 /media/debian-home-partition # debian /home partition

I want, that debian grub2 stay the main bootloader. Debian-loader is in /dev/sda and my bionic grub2 in /dev/sdb

I want that both are untouched.

yes, I could tell ubiquity to install grub2 on a USB stick and after that I can remove the bootcode.

I read online the manpage from ubiquity and do not find any option for that?

My systems are in bios-mode.