What is the relationship between Iago and Roderigo in Othello?

In Othello, Shakespeare begins the
play with two foils, Iago and Roderigo at night: one hidden and the other in plain
sight.  The former is a villain, the latter a fool, but both are be dark lords of
misrule.

Here's what they have in
common:

• Both are targeting Desdemona (Roderigo
  as thwarted love; Iago as revenge bait against
  Othello)


• Both are jealous of
  Othello


• Both are expose Desdemona's secret elopement to
  Brabantio (Roderigo publicly; Iago privately)


• Both lose
  their battle against Othello in Act I (the Duke condones the
  marriage)


• Both seek revenge in Cyprus (away from the
  Duke's protection)


• Both are id-based
  characters (exhibit child-like behavior and seek immediate pleasure: Roderigo=lust;
  Iago=suffering of others)

Here's how they are
different:

• Roderigo is public in Venice and
  private in Cyprus; Iago is private in Venice and public and private in Cyprus: (as such,
  they are two halves of the hidden, dark side).


• Roderigo
  is pure passion (he threatens suicide); Iago is calculated passion (he wants only to
  wound others)


• Roderigo pays Iago.  Roderigo is the
  "money" and Iago is the "purse."


• Roderigo is a fool.
   Iago admits, "I am not what I am" to Roderigo, and yet Roderigo continues to believe
  and pay him


• Roderigo thinks his using Iago for his dirty
  work, but Iago is really using Roderigo to be his scapegoat.  Iago uses Rogerigo to
  expose Cassio.