Brass Knuckles: These close combat weapons are designed to fit comfortably around the knuckles, narrowing the contact area and therefore magnifying the amount of force delivered by a punch. They allow you to deal lethal damage with unarmed attacks. You may hold, but not wield, a weapon or other object in a hand wearing brass knuckles. You may cast a spell with a somatic component while wearing brass knuckles if you make a concentration check (DC 10 + the level of the spell you're casting). Monks are proficient with brass knuckles and can use their monk unarmed damage when fighting with them.
If a monk dual wields brass knuckles, is the monk's 2nd attack an offhand attack?
No when a monk flurries they can interpose any attack they are capable of that is compatable with a flurry. If you had 6 attacks kama nunchuk kick headbut kama nunchuck is possible none of them are offhand attacks.
If you have one brass knuckle you can use it 6 times.
However they no longer add the monk damage.
I know this can be confusing, but apparently monks have two separate abilities that say they do not have offhand attacks(under certain circumstances)
With the flurry of blows, a monk can use any monk weapon or unarmed strike in order to get what is basically TWF. But they do not have offhand attacks, and they can in fact just use a single weapon if they wish (so dual wielding is actually wasting money)
The ability that Sarrah is likely referencing though is the unarmed strike ability.
Unarmed Strike
At 1st level, a monk gains Improved Unarmed Strike as a bonus feat. A monk's attacks may be with fist, elbows, knees, and feet. This means that a monk may make unarmed strikes with his hands full. There is no such thing as an off-hand attack for a monk striking unarmed. A monk may thus apply his full Strength bonus on damage rolls for all his unarmed strikes.
Thus, even if you only dipped monk and used regular TWF (which I usually go ranger for for a nice combo, since they can also get greater magic fang), you do not suffer offhand penalties. Which can be nice with a good strength bonus and power attack, since you are getting x2 total rather than the normal x1.5 total.
Unfortunately, Sarrah, various rules threads already have dealt with brass knuckles and the like (you can look them up yourself to verify). Essentially, brass knuckles and the cestus both are merely light monk weapons, and they have nothing to actually do with the unarmed strike ability. So, sorry, your offhand attacks are dealing x1/2 strength and power attack.