There are several mobos that support dual athlons on the market today. Tyan was the first to offer one I believe, but it was quite expensive (<$400 IIRC) and aimed at the server market. Now with the advent of the 760MPX chipset by AMD more desktop mobos are becoming available.

Here's a link to the tyan board, and here's a recent roundup of mobos using the 760MPX chipset done by the guys over at firingsquad.com