You shouldn't need to select a drive.
All you need is the command
After entering that command, drag your ISO to the terminal (which will add the ISO path to your drutil command). That will complete the command, and you then press enter to run that burn.
The drutil list command might help you decide how to complete the command, but to just get the ISO burned to a DVD, you could temporarily unplug the lower drive, so only one drive is connected. I don't even think you would need to do that, as whichever drive that has a blank DVD+R in the drive should accept the command.
I think that you are also incorrect about Boot Camp and Mojave, as I have burned a bootable Win 10 installer DVD from that just recently. You STILL have to use the 1803 version (the 1809 version will only burn from Windows, as far as I know), and as long as you have the 1803 version, you can also create a bootable USB flash drive for Win10. Again, you have to use the older 1803 version. The flash drive needs to be formatted as fat32, or ExFAT. I don't know where you might have seen some info that Mojave Boot Camp is "not supported" That's just incorrect. I have used the BootCamp assistant to install Win10 on 2 different Macs, both running Mojave, within the last couple of months, so your idea is simply wrong. Boot Camp IS limited, because you have to be careful which version of the Win10 ISO you use, but Boot Camp itself does its work of making a partition that is ready for a Windows install, and also downloads the Windows support files. THOSE are outdated, I think, but easy enough to fix through Device Manager, and the Apple updater, once you have Win10 installed.