Well firstly I would caution you to ensure that they are kept very separate. A common switch or VLAN could panic if it seems to see the same MAC on two ports.
It's been a while since I have done this sort of thing (Solaris 2.6!) where we labelled the NIC with a MAC based on the sub-area-node definition for a VTAM connection to an IBM mainframe. I think we actually set the Token Ring NIC with that and the Ethernet NIC with something else. I think the licence was based on it, so you might need to be careful for that too. I regret that I don't have any Solaris left
The 'It cannot be done!' is a lie, for some architectures at least. Some lesser Operating Systems can't cope though. I think you have it about right though.
Why do you find it necessary to spoof a MAC address?
What are you trying to achieve? To clone a Solaris system for production you generally don't need to alter the hardware MAC address because it has no bearing on anything. You might need to change the hostid of the system but not the MAC address.