Usually riding counts for running and vice-versa as far as endurance goes, but not as far as muscle-specific development goes (you'd also need to ride farther to count for the same running endurance, which actually benefits your situation since you're training for a MUCH longer ride than you are run). You still want to do both, but if you're doing a long ride/long run, it will benefit both disciplines.

For triathlons, I do long rides one day and long runs the other day - but it's different because you plan on doing them in succession, so you kind of "want" that "legs are tired" feeling. You could put a rest day inbetween the two if you wanted to do them both in the same weekend. Something ilke:

Mon - short ride
Tues - short run
Wed - short ride (or intervals, hills, etc)
Thur - short run (or intervals, hills, etc)
Fri - long ride
Sat - off
Sun - long run
Mon - short ride
etc

I find that riding after a long run is easier than running after a long ride.

I have done it the other way, too - rest day before your long ride/run days rather than after. I've never been able to split them because I've always been triathlon training and I need to do it on the weekends.

I also agree that you could consider a short ride/long run and short run/long ride variation that alternated every other week. Since you're building endurance either way and don't intend to do them both in one day (or back to back), you'll be benefiting yourself a lot. It would suck to over-train and miss both targets - or be too exhausted to do the half marathon after your ride.