Which are the advantages of splitting the developer's time between two projects?

I have two projects, with identical priorities and work hours demand, and a single developer. Two possible approaches:

1. Deliver one project first.
2. Split the developer's time and deliver both later.

I can't see any reason why people would choose the second approach. But they do. Can you explain me why?

It seems to me that this decision often comes down to office politics. One business group doesn't want to feel any less important than another, especially with identical priorities set at the top. Regardless as to how many different ways you explain why doing both at the same time is a bad idea, it seems as though the politics get in the way.

To get the best product to the users, you need to prevent developer thrashing. When the developers are thrashing, the risk of defects and length of delivery times begin to increase exponentially.

Also, if you can put your business hat on, you can try to explain to them that right now, nobody is getting any value from what the completed products will deliver. It makes more sense for the business to get the best ROI product out the door first to begin recouping the investment ASAP, while the other project will start as soon as the first is finished.