User.from(
  Arel::Nodes::TableAlias.new(
    Arel::Table.new('users')
      .project("id, name")
      .union(Arel::Table.new('another_users').project("id, name")
    ), 'users'
  )
)

SELECT  "id", "name" FROM ( SELECT id, name FROM "users" UNION SELECT id, name FROM "another_users" )