This should be mind-numbingly simple and basic, and yet I couldn't find how to do it, either in "man ifconfig", in the OpenSolaris docs site, or Googling in desperation. I'm using OpenSolaris 2009.06.
When I try ifconfig e1000g0:0 10.10.10.1 netmask 255.0.0.0 up
all addresses go kablooie instead. This happen whether the network is in "automatic" or "manual" mode.
Alternatively, I could use a way to set up two static IP addresses too.
Try
ifconfig e1000g0:1 blah...
orifconfig e1000g0 addif 10.10.10.1/16
instead - IIRC the zero alias/logical interface is special on Solaris.