One might consider a Orb/Meteor in stead if one wants to combine fire and ice.
As for blizzard, I think I would just max the blizzard's synergies, rather then weakening it's effects to try and go multitree.
Multitree was viable back in 1.0x, but in 1.1x it is ill-advised to go multitree-ing with skills such as Blizzard which can be far more effective as pure builds and would probably lack killing-power in a double element build.
If you really wish to continue down this path, I think a combination with fireball would yield the best results(I didn't run it trough the calc though).
I can imagine combining blizzard with a spell you can direct would work quite well. Though the lack of skillpoints available makes it so you have to get some serious equipment.