Why Do Two Reed Blades Vibrate Face to Face on an Oboe?

The “double” in double reed does not mean two pipes or two notes. It refers to the two thin cane blades facing each other at the top of the oboe. As the player supplies air, pressure and the spring of the cane make the blades open and close rapidly, turning a continuous breath into a periodic airflow. That vibration then interacts with the oboe's narrow, gradually widening conical bore to form the pitch and tone we hear. A clarinet instead uses one reed vibrating against a fixed mouthpiece, so its starting conditions and relationship with the bore are different.

The two blades are not there to double the volume. Together they form an extremely sensitive variable opening. Cane thickness, the scraping profile, moisture and lip pressure can all change response, intonation and the balance of overtones. The entrance to the oboe's bore is also very narrow, so relatively little air passes through at a time. The player's problem is often not running out of air, but controlling airflow against substantial resistance and releasing air that has accumulated.

When listening to an oboe, notice how a sustained note can unfold from a narrow centre. Fine playing does not reduce the instrument to a fixed “nasal” colour; it changes brightness, weight and direction while preserving a tonal core. Because the reed responds so closely to breath and embouchure, an oboe solo can sound both concentrated and continuously inflected, rather like a speaker adjusting vowels within a sentence.

https://philharmonia.co.uk/resources/instruments/oboe/
https://www.yamaha.com/en/musical_instrument_guide/oboe/mechanism/