CREATE TABLE entity_ones (
  id INTEGER,   
  -- whatever goes in this table
);

CREATE TABLE entity_twos (
  id INTEGER,
  -- whatever goes in this table
);

CREATE TABLE comments (
  id INTEGER,
  author VARCHAR(128),
  content TEXT,
  -- etc
);

CREATE TABLE entity_one_comments (
  entity_one_id INTEGER NOT NULL REFERENCES entity_ones(id),
  comment_id INTEGER NOT NULL REFERENCES comments(id),
  PRIMARY KEY ( entity_one_id, comment_id )
);

CREATE TABLE entity_two_comments (
  entity_two_id INTEGER NOT NULL REFERENCES entity_twos(id),
  comment_id INTEGER NOT NULL REFERENCES comments(id),
  PRIMARY KEY ( entity_two_id, comment_id )
);