Hi Christoph,
You have see this in different way, two DB inside same instance is always better than two instances. Generally the Dev and Test/Demo system will be on different instances.
I have read in HCP also SAP offers this feature, check the below link:
http://help.sap.com/hana/SAP_HANA_Server_Installation_Guide_en.pdf
Thanks & Regards
A.Dinesh