OK, took a careful look at both maps, and here's what I can deduce.
The half and full courses are the same until just past 10 miles, which is on Belmont Ave. just after you pass Memorial Hall.
The half marathoners will then take the FIRST right onto Landsdowne, while the marathoners will continue on to the SECOND right onto Horticultural. Then (and here's where things could get SNAFU'd) the two courses rejoin on Landsdowne, almost immediately after the halfers have taken that turn. So essentially, the marathoners just do an extra loop on Horticultural (one of the nicer stretches on the course, BTW) and then come right back to the half course.
The organizers are going to have to be very diligent about making sure marathoners don't make that first right and cut about 0.1 from the course, inadvertantly or deliberately. That, and the HM turnaround 175 ft. from the finish, lead me to have some reservations about how this is all going to work out. I'm holding my breath.