Is Multi-Boot Possible in Android?
Yes it is possible. As Irfan mentioned there are hacks which allow, essentially, dual booting with a Linux distro, the list of which is rapidly expanding.
Dual booting native Android ROMs is possible as well. Dual Boot Patcher app by xda developer Anik_khan allows just such a scheme. Additionally devices with A/B partitions certainly can dual boot(although /data is shared). It is also possible to configure a ISO within an  Android device that can be bootable via USB, say for a laptop...