There's a neat patch at https://sourceforge.net/projects/bash-gchistory/ that implements a shared history when running multiple instances of bash. I modified the patch a bit to apply cleanly against bash-3.1_p16, and added a 'global-history' use flag to the ebuild.
Created attachment 93595 [details, diff] Patch to the 3.1_p16 ebuild to add the use flag and apply the patch
Created attachment 93597 [details, diff] Global history patch based on gchistory v0.1.6 updated for bash-3.1p16
i'm pretty sure upstream hasnt seen this ... it needs to sent there first
I doubt upstream would want to mess with this. Oh well, was worth a shot :)
you'll never know for sure if you dont ask ;) we have enabled the history append option now in bash ...