Do I have to use both hands for this sign?
While the two-handed version is standard and very clear, it is common in casual conversation to use just one hand, especially if your other hand is occupied.
Why do I need to furrow my eyebrows?
In ASL, facial expressions act as grammar. Furrowed eyebrows indicate a 'wh-' question (who, what, where, when, why), letting the person know you are asking for specific information rather than a simple yes or no.
Can this gesture mean anything else?
While this specific sign is understood as 'what', the open-handed palms-up gesture can also be used in contexts like 'well?' or 'what's up?' depending on the exact movement and facial expression.