amd64 cpus are backwards compatible and can run 32bit stuff, too.
if you run a 64bit kernel, you can still have a 32bit userland in linux.
if you run a VM you can run either 32bit or 64 bit inside without issue, too.
some linux distros allow you to run under the 64bit kernel both 64bit and 32bit userland at the same time with a complicated mechanism called multilib. the resulting complexity is gross.
you can have the same effect by just running a second userland inside a chroot, i do this all the time. so i have multiple linux userland root filesystems instead of multilib.