As I recall, Toyota put in some fancy electronics to determine which antenna signal to use. Far more complex than a simple splitter like the one you linked to. I don't know what the reasoning behind that was, however, knowing Toyota, there was some thought put into it.
Personally, I've never hooked up both. On my 7M car, I use the retractable power antenna. On my 2J car, I use the hatch glass antenna. I get great signal in both cars, so I've never worried about it.
I'm not sure buying off ebay is neccesary... I've never found a car stereo install bay that *doesn't* have a shitload of these, and I'm pretty sure I've never paid over $5 for one. You should literally be able to walk into any car stereo shop and ask the installers for one, they'll probably trade you straight across for a hot dog and a red bull.