Mark,
Should be able to do both easy in a day. I went from Savannah to St Augustine in a day, seeing Tybee, Cockspur, St. Simons, Amelia Island & both Jacksonville lights. If I remember, ought to be able to go from Tybe to St Simons in a couple of hours.
We'll be not too far behind you. Leave tomorrow afternoon for Savannah and then to Charleston. Will also do the St Simons loop one day.
Have a fun trip.
Dave