Looking at the course map, it seems theoretically possible to stand between the two directions of travel around mile 14/21, just west of St. James's Gardens. However it's hard to believe that would be allowed for security reasons, so I'm thinking that it should work to watch them turn onto Narrow St. (mile 14), then after both women and men have passed, walk north to see them again from the east side of Butcher Row (along St. James's Gardens) around 21.34 miles.
Here's the course map, for reference:
Has anyone done this and can let me know what watching from that area is like?