Secondary Arms.
You have two slightly smaller secondary arms below your primary pair of arms. The secondary arms can manipulate an object, open or close a door or container, pick up or set down a Tiny object, or wield a weapon that has the light property.
So in this example, let's say a Thri-Kreen has a Longbow in one hand, and a shield in the other. Using its secondary arms, could it still operate the Longbow? I assume it can under the "manipulate an object" feature, but how does it consider the "wield a weapon that has the light property" since you don't need to hold a longbow in two hands, you just need two hands to operate it. But a Longbow def ain't a light weapon, but you're also not using the smaller arms to wield a longbow either, the bigger arm is wielding it.