Two attack for ST 10 i one for ST 11? I am assuming you are talking about an infant, which has RoA 2. Then, on Paroxysm:
Two dice for st [model] and one die for St [model+1].
So, you roll 2 dice for 10 and one for 11. It helps using different color dice.
I bought a pink one for the tail strike, and it always hits.