function two_spins_build_up_H1H2 = two_spins_build_up_Fun(lambda_p,lambda_m,sigma_H1H2,time,sigmaEffScaling)

delta_lambda            = lambda_p - lambda_m; 
two_spins_build_up_H1H2 = - sigmaEffScaling(2) * sigma_H1H2 * ( exp(-lambda_m * time) - exp(-lambda_p * time) ) / delta_lambda;    
