It may seem counterintuitive, but similarities make multitasking harder, according to Poldrack. Before getting to the bottleneck, mental processes often originate in different parts of the brain. Pick two from the same area and they can become garbled. For example, you may enjoy reading with music on, but if you time yourself, you'll likely turn the pages much faster when there are no lyrics to distract you from the text.
Try to match projects with different modalities—like reviewing a report while on the stationary bike.