[polyrand]

> Any downside to this approach?

None that I can think of unless you need foreign key constraints between both.

> Are JOINS slower across two files than across two tables in the same file?

I was recently debugging as slow JOIN with ATTACH'ed databases and the query plan looked the same as when both tables were in the same database. I don't think it makes any difference.

But in these situations, the solution is measuring and benchmarking for your use case.