Neither Logic samplers seems to not handle properly REX files at the moment. If you drag REX file to audio track, Logic asks if you want to render it new audio file, if you say yes, then you can drag that region to both samplers.
I had lot broken .esx files, because few years ago I had to copy samples to different drive, with different name.
I don't know if it is only me, but now all my previously broken .esx sampler instruments find automatically their corresponding samples!
If you own Novation Launchpad, Launchpad MK2, Launchpad Mini, Launchpad Mini MK3, Launchpad Pro, Launchpad S or Launchpad X, then there is nice surprise waiting for you in Logic Pro X 10.5 .
Well done Apple & Novation.