In[]:=
Manipulate[Plot[{PDF[NormalDistribution[-d,1],x],PDF[NormalDistribution[+d,1],x],PDF[NormalDistribution[-d,1],x]+PDF[NormalDistribution[+d,1],x]},{x,-5,5}],{{d,2,Sigmas},1,3,0.1,Appearance->"Labeled"}]
Out[]=
​
Sigmas
2
-4
-2
2
4
0.1
0.2
0.3
0.4