The description is lacking some details of your installations, like: where the boot loader is (disk A or B), and what boot loader is used? What was the order of OS installations that you have carried out (which OS first)? (EDIT: no picture with an error, no picture with round brackets you are referring to.)
However:
In a nutshell, all systems that you want to have bootable on your computer (using one distinct BIOS setup/setting) shall be configured on a distinct boot-loader on a disk chosen as booting disk in BIOS. This way you should expect a boot menu on power-on where you choose the system to continue with.
Additionally:
- The binaries of the operating systems on that computer can be located on any disk (i.e. the same disk, different partitions, or different disks).
- Linux may be perceived easier to install its bootloader which also includes another (Windows) OS start-point, than configuring the dual boot in Windows. And this may be subjective.
If you installed Windows on one disk, then installed Linux on another disk, separately, with no common configuration of the boot-loader that the computer would use for booting, then you will have to manually choose in BIOS which disk you want your computer to boot from each time you want to boot the other OS. Tedious this may seem, it is a possible (I don't think it is the first choice for the most though) way to go if the installations have their separate boot loaders on separate disks.
