These answers, though, don't really answer my question. I didn't ask how I should change what I'm doing to fit Apple.
Note that we're not trying to make you change to "fit Apple." Nothing about our answers is Apple-specific, except for the fact that we're discussing Apple products. You could achieve the same in the Mac version of Picasa, I believe, and the advice would be equally relevant on a PC. You're right that these don't answer the core question, but it's often the case in forums like this that the core question isn't really the question that needs to be answered, so you must not be offended when someone tries to help correct what they see as a flaw in your workflow.
I asked if Apple has, in my mind, a very basic and logical function to merge existing folders from 2 different drives with the same name.
As said, the answer to that is no. Of course, it's my opinion that if you're frequently relying on merges, you've got a very dangerous workflow going. Once upon a time, I used to try to keep certain folders on two laptops synced, using [Synk Pro|http://decimus.net/Synk>. Because I was modifying files on both machines daily, I caused a lot of my own headaches and lost a lot of data. This was through no fault of Synk's, but because such a workflow is inherently dangerous. It is
far better to keep
one working copy only. If you need to share it between machines, put that data in "the cloud," or on a flash drive, or on a portable external hard drive. Your solution of putting your photos on a drive connected to your wireless router and sharing it between two machines sounds absolutely perfect, and avoids the merge issue entirely.