A foreign key can only reference one table, as stated in the documentation (emphasis mine):
A foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between the data in two tables.
But if you want to start cleaning things up you could create a members
table as @KevinCrowell suggested, populate it from the two members_company
tables and replace them with views. You can use INSTEAD OF
triggers on the views to 'redirect' updates to the new table. This is still some work, but it would be one way to fix your data model without breaking existing applications (if it's feasible in your situation, of course)