For a developer, I think multitasking between two tasks (a higher priority task and a lower priority task that can be worked on while the higher priority task is blocked or waiting or otherwise stuck) is manageable and can work well. Beyond two the task switching cost gets too high in a hurry. It's also not good if management/customers are insisting that both tasks are very high priority, it's too stressful to juggle…
Two things having very high priority means really, neither are. I've reminded many a manager of this in the past.
...that just means you don't prioritise any crime.