On Netapp for example (having 2 hbas and 2 ports per controller) you will have 4 active paths, but only 2 active (I/O). Each lun here can be accessed through the owner controller only, so for each lun you would have 2 working paths, i.e. both hba will do the job through the one controller.
When the one controller head will go down the work will be continued through the second controller.
Having more hbas and more ports per controller will give you even more working paths.
I'm not sure now if there are any boxes in the market that can do that through all the controllers (maybe 3Par, EMC Symetrix?).
Reg
Christian