In the formula you are using, the joint standard deviation is wrong.it should be
T_high=(μ1+μ2)./2+k1∗sqrt((σ1^2+σ2^2)/2);
T_low=(μ1+μ2)./2-k2∗sqrt((σ1^2+σ2^2)/2);
As you treat all 18 pixels as belonging to the same distribution, why not use the following
v=[v1(:);v2(:)];
μ=mean(v);
σ=std(v);
k1=k2=1;
T_high=μ+k1*σ;
T_low=μ-k2∗σ1;