This approach can never be trusted. You can't have data of different security levels on the same media, circuitry, network,..., what ever. Even if you could create secure software - yeah right, this would not protect against a hardware malfunction. Any number of failures could cause secure data to be available at a lower level. Something as simple as scrapped wires touching, or a staple shorting two contacts could echo data into a different virtual machine. Of course such a "malfunction" could be introduced by any person with physical access to any part of the system/network that "sees" this data.
Ken