From a best practice perspective, even in non-HANA landscapes, SAP does not recommend to have more than one DB on the same logical instance (i.e. on the same OS). What you could do today is to share the same, say, Oracle instance between more than one application server (each one with its own Schema). That's what is gonna be available in SPS9.
Regarding LPARs, for the HW providers that support it, it's already possible to do it today.
I have seen that with Fujitsu & Hitachi machines.
As for IBM/AIX, when HANA is available on Power (planned for SPS9 as well), you'll also be able to use LPARs on the same physical machine for different HANA instances.