We have an old Key West home (pier construction) with joists that are 2"X 5 1/4"(real size Dade pine)spaced about 24" apart.They are notched to fit the sills about 4 1/2 inches. I would like to add joists between them - they would be 1/2 the width of the house and rest on the center beam. However fitting anything with a 18 inch craw space is a problem without major surgery. I am wondering if I could use a kind of sandwich consisting of a 2x4 at the center and 1x6's on each side.

Any help would be appreciated