UPDATE:
It seems there is version of Xenon 2 on 1 Disk in some releases of this as I've just obtained a boxed version and Xenon 2 is the 1 disk version with Speedlock and MFM tracks!
So HOL should list both 19 and 20 disks
Also, there is disk with both Krypton Egg & Eliminator on it but also has Eliminator on a separate disk too!