I am thinking about making a "Monk" glaive and I was wondering about the Wields Two Weapons At Once Focus.
Are unarmed weapons, like fists and feet, considered light weapons?
"Wields two weapons at once" is a focus, not a descriptor -- and there's another one, "Needs no weapon" (Cypher System p. 128). Under the Cypher system rules (of which Numenera is a subset), you can't use "two weapons at once" simultaneous with "no weapon", because a character can have only a single focus. Further, unarmed fighting in general, regardless of focus, would imply use of both hands (boxers, the most restricted unarmed fighters, use both fists; every other unarmed discipline uses the whole body), but is treated as a single attack.
Therefore, as I read it, no, you can't treat unarmed fighting as light weapons for "Wields two weapons at once" purposes. Unarmed fighting has its own handling covered under "Needs no weapon". Some types and descriptors can, however, gain "No need for weapons" as a Tier ability -- this would be one of several alternatives in that case, so while everyone who "Needs no weapon" fights unarmed, not everyone who has the option to learn "No need for weapons" will actually do so.
In the Numenera corebook (pg. 80) 'Unarmed' as a weapon (which specifically calls out kicks and punches) is listed as a zero-cost light weapon. In fact, all the descriptive texts for tier abilities like 'Needs no weapon' throughout the book explicitly mention that that ability upgrades unarmed to medium weapon status from light weapon status.
So in Numenera, yes, unarmed attacks are indeed light weapons. As for whether or not you can use unarmed attacks for "Wields Two Weapons at Once", the focus flavour text assumes actual weapons, but doesn't require it. However, it really depends on how you interpret 'unarmed' as a weapon.
If you go with the camp where using unarmed attacks in any form is collectively considered a single weapon, then it's impossible to use it with the focus, as you can only ever have one of them. If you go with the camp that feels each fist, foot, knee, elbow, forehead, etc are all separate weapons in your unarmed arsenal, then yes, you should be able to use it just fine as far as the rules are concerned.