From the Rufus FAQ...https://github.com/pbatard/rufus/wiki/F ... ot_support
(He's a bit shouty and full of it, but from bitter experience much of what he writes is true...).
...Then again, if you really insist on using Rufus as base for multiboot, you might be interested in this tutorial from our friends at RMPrepUSB... or you might as well use Ventoy (which is most likely what you are looking for) or RMPrepUSB or Easy2Boot or YUMI as there already exist quite a few solutions to perform what you seek.
Alternatively, you can do what the multiboot pros (actual sysadmins) do, and invest in a hardware device that is dedicated for multiboot support such as an IODD drive. This is because a hardware solution is the ONLY GUARANTEED way to ensure that multiple ISOs from a single drive can boot as intended. The reason why a hardware solution is needed is that, due to the contradictions of incompatible boot loaders and ISO → USB conversion, the only solution that works reliably is to present the USB device as a virtual CD or DVD ROM, and then present the selected ISO image as a CD or DVD disc mounted in the virtual device. But of course, any software solution that tries to do that after the USB has already booted as an HDD drive (which cannot be avoided) does have to make major compromises to try to pretend that your multiboot image was actually booted as a CD or DVD. However, since the BIOS or UEFI firmware knows full well that that wasn't the case, a software solution can NEVER reliably guarantee that the ISO will boot and work properly and the only way to address that issue in a pure software solution to modify the ISO content, either at runtime or prior to boot, which must be customized for each ISO type and which, as the developers of Ventoy explain, is hard work because there are so many different OS distros and so many special cases. As such, if you really are serious about multiboot, you will invest in a hardware solution... Cont.