Why is PostgreSQL blocking creation of a new table on update on foreign table?
If you are executing both statements in a single transaction, they won't lock each other.
If you execute the statements in different transactions, one will have to wait until the other is done. The reason is that foreign key relationships are implemented by system triggers in PostgreSQL and creating a trigger on tablea requires a SHARE ROW EXCLUSIVE lock on the table.
If you look at the lock compatibility table, you will see that that lock conflicts with the ROW EXCLUSIVE lock required by INSERT, UPDATE and DELETE.
Make sure that your UPDATE are running in short transactions, then that won't be a problem.