Hey @SDL17 ,

If you want to use two range extenders, we recommended connecting both range extenders directly to the router , not extender wirelessly connects to another extender ( for the performance and stability of the 2nd extender) 
Recommended Topology 1: RE1 )))(( Router )))(( RE2

May I know what happened when you try to add the second extender?

You could unplug the first extender when set up the second extender, after the set up process, plug both extender and see if both REs show up.