V2.2 is a more uncluttered version and retain database context for up to 5 opened main windows. There is an option to only show database list. The separator lines don’t have the section title now.
PS: This script might become obsolete very soon 
. See Feature request DEVONthink Pro 3: 3 pane view & tabbed windows
Demo
A search list in DB1, invoke the script and switch to DB2.
In DB2, switch back DB1 again.
The search list in DB1 is retained.
Limitation: the exact selection in the search results can’t be retained. Only the search list is retained.
In DB1, switch back to DB2
The list and selection is DB2 are retained
The uncluttered list, if views are also enabled.
EDITED 2019.09.26: the latest version that supports multiple main window’s database context retentions is attached in post#2.