mix = sample(0:1, 1000, rep=T)
 x = mix*rnorm(500, 100, 15) + (1-mix)*rnorm(500, 150, 20)
 mean(x);  sd(x)
 ## 126.7677
 ## 30.89346

shapiro.test(x)

 ##        Shapiro-Wilk normality test

 ## data:  x
 ## W = 0.9703, p-value = 2.043e-13

mix = sample(0:1, 1000, rep=T, prob=c(.20, .80))
 y = mix*rnorm(500, 100, 15) + (1-mix)*rnorm(500, 115, 20)
 shapiro.test(y)

 ##   Shapiro-Wilk normality test

 ## data:  y
 ## W = 0.9966, p-value = 0.02869