There is a significant overlap between physics and chemistry.
For example: Explanation of "thermite vs ice" explosion
Therefore I suggest that questions tagged "chemistry" on physics.se be displayed and editable on both chemistry.se and physics.se. The reverse would also be beneficial: questions on chemistry.se tagged physics could appear here as well as there and be editable by both communities.
A question immediately arises, should we collaborate on other tags? I would suggest that only tags involving the word chemistry or physics respectively should be cross-communal projects.
There is a question on SO meta: Asking on Multiple Sites: "Soft-Migration" which points out how it is mostly the sciences that would benefit from this feature. I guess that means it is "too specific," since we are relatively small. Democracy baby!