My hand wavy math.

One person dedicated to a single project / task baseline 100% total output.

One person split between two tasks / projects that require context switching, 70% total output at best, perhaps 50% total output, a loss of 50%.

One person working on a single task that they’re passionate about / intrinsically motivated to complete: 150-500% output.

When you have one task / project to think about and focus on, it engages your brain: sleeping, showering, walking, your subconscious is processing.

Adding just one extra project / task that requires a context switch mostly kills this subconscious processing mechanism.