Actually this is not the case the the lenses I think he is askng about. I assume that he is asking about the Tamron 60mm f/2 and the Tamron 90mm f/2.8. These have nearly the same working distance since the Tamron 60mm suffers from much less focus breathing then any other macro lens. Meaning at 1:1 it gives nearly the same working distance as the 90mm.
I guess that makes sense considering how much the older 90mm Tamron extends when at 1:1. The newer version is internal focus though, so I'd have thought that it would have a working distance advantage.
In reality, it's the other way around. Instead of extending the lens, the focus system works halfway like a zoom, reducing the focal length to focus at shorter distances while keeping the extension unchanged.
I don't believe any SLR manufacturer reveals the actual focal length at 1:1 or other magnification ratios, but my own test shows that Minolta's 200 mm macro has its focal length reduced to a mere 105 mm at 1:1. Still, the ringflash-to-subject distance is just under 25 cm, while with my 50 mm macro it's less than 5 cm, so there's definitely still an advantage when shooting insects and spiders.
Ceterum censeo soleam calidam ISO esse delendam.