I've been wondering if I could find Split image focusing screen for my 5D mark II.
Brightscreen
http://www.brightscreen.com/styles.html
Haoda
http://www.haodascreen.com/Pages/Canon5D.aspx
focusing scree.com
http://www.focusingscreen.com/
and of course by far the least expensive is Canon Ec-B, with many, many suppliers:
http://atinyblip.com/articles/photography/eos-focusing-screens/
All split screen focusing screens will impact your exposure, as the split is always in the center of the frame. Canons general advice for this choice is DON'T use center spot metering and use center weighted rather than evaluative metering. If you use fast glass (f2.8 or faster) the exposure change will likely be very small while for f5.6 lenses the exposure change can be pretty significant, and the viewfinder brightness in the split area of the screen can be dramatically decreased under dim conditions.
For tripod based work, Live View manual focus with 5-10X in-camera magnification, is a much better solution. This is my preferred MF approach, especially for telephoto or tillt/shift lens use. Use the joystick to move the magnified area about in the frame to optimize focus on what is critical; the same LCD green rectangle is used to establish exposure.
Mike K