...if the secondary effect chance is p (for p: 0<p<1), then the chance of having the effect occur at least once in the two hits is 1-(1-p)^2 = 1-1-p*p+2*p = 2*p-p^2 = p*(2-p). for example, body slam, p=.3, with parental bond yields p' = .3 * (2 - .3) = .3 * 1.7 = .51, where serene grace would...