First prewarp the frequency of both poles:
K1 = tan(0.5*Pi*20.6 / SampleRate) // for 20.6Hz
K2 = tan(0.5*Pi*12200 / SampleRate) // for 12200Hz
Then calculate the both biquads:
b0 = 1
b1 = 0
b2 =-1
a0 = ((K1+1)*(K1+1)*(K2+1)*(K2+1));
a1 =-4*(K1*K1*K2*K2+K1*K1*K2+K1*K2*K2-K1-K2-1)*t;
a2 =- ((K1-1)*(K1-1)*(K2-1)*(K2-1))*t;
and:
b3 = 1
b4 = 0
b5 =-1
a3 = ((K1+1)*(K1+1)*(K2+1)*(K2+1));
a4 =-4*(K1*K1*K2*K2+K1*K1*K2+K1*K2*K2-K1-K2-1)*t;
a5 =- ((K1-1)*(K1-1)*(K2-1)*(K2-1))*t;
Now use an equation for calculating the biquads like this:
Stage1 = b0*Input + State0;
State0 = + a1/a0*Stage1 + State1;
State1 = b2*Input + a2/a0*Stage1;
Output = b3*Stage1 + State2;
State2 = + a4/a3*Output + State2;
State3 = b5*Stage1 + a5/a3*Output;