Well, I have to say, the custom of pulling a wishbone seems to date back to pre-Roman times, and is pretty much found throughout European culture, although what bird bone is used sometimes varies.
Many of the anecdotes say that breaking a bigger piece meant the holder's wish would come true, so if it broke exactly, I'd say that can be interpreted two ways. One might be that you both have an equal chance of the wishes coming true, and the other might be that neither wish will come true.
There doesn't seem to be much commentary on this particular scenario.
