Shorting both is not a pairs trade. Short one, long the other is a pairs trade. With these instruments, all you have to do is go long the undervalued, or short the overvalued if you want. Or do half long under, short half over.
In your earlier post you quoted, "They must be negatively correlated by -0.9 to -0.99 to qualify as a pair trade" Then how you will take both Short and Long positions?