I was looking up Monk's Unarmed Strike for another answer and was wondering. If a power, such as Twin Strike requires you to be wielding two weapons, are you effectively wielding two unarmed strikes, or must you use some other weapon in either your main or your off hand?
This, as far as I've gleaned from other situations, DMs, threads, and the rules, is a house rule question.
According to the letter of the rules, you are wielding one weapon, Monk's Unarmed Strike, that you can make multiple attacks with if you have the multiple attacks to use. Not two weapons. Game balance wise, I see zero issue with allowing this, and have never heard of a DM who did not allow two monk hands to be used as two weapons for feats – as long as you have both hands empty and usable as monk's unarmed strike weapons when using the feat.
Treat Unarmed Strike as a single weapon for 1 hand. When using it for Two-Weapon Fighting, apply the normal TWF penalties. I would consider the other hand to be a light weapon for rules purposes and apply the corresponding penalties in-line with the TWF feat. -4/-4 in this case.