Would this work

[Rass]

{spine bitter} + {Mage slayer} = double damage before actual combat damage if it was blocked?
Also, what would happen if you threw in an {assault strobe}

[Goblinking]

I don't think that doubles it it's still just hitting the once

[scarsabrex]

you get 6 poison. 9 with doublestrike

[Rass]

Ah, so {giant growth} would be better

[BlackJester]

As soon as you declare {Spinebiter} as an attacker, it would deal 3 poison to your opponent, because that is when the trigger happens, (doublestrike won't affect this.) Then, during the combat damage step, you would deal 3 poison (6 with doublestrike).

Like scarsabrex said, with a little more detail. 

Overall, I like these together.  sneak in one more poison, or pump it's power by one and you're "good to go"!