How to put two arrows in different directions with text over and under them

Question

I found here in stackexchange some latex code to make two arrows in the same direction with text over and under it, by using the package extpfeil.

\newextarrow{\xrightrightarrows}{{20}{20}{20}{20}}{\bigRelbar\bigRelbar{\bigtwoarrowsleft\rightarrow\rightarrow}}

Now I want the bottom arrow to point left, and I don't know how to change this. I tried with this:

\newextarrow{\xleftrightarrows}{{20}{20}{20}{20}}{\bigRelbar\bigRelbar{\bigtwoarrowsleft\rightarrow\leftarrow}}

But it doesn't work, since all it does is this.