The general workflow below should work, note that some of the nomenclature I used may not be what InRoads calls it:
1. Lets call the original surfaces S1 and S2. Create a third surface that compares them (perform a cut fill). Lets call this S3.
2. color fill areas of cut and fill in S3. If S1 was the base/datum and S2 was compared, the fill areas will be where S2 is higher than S1
3. generate the "zero" elevation contour on S3 from step 1. These contour lines should ride the limits of the cut and fill limits color limits generated in step 2. discard the zero contour lines around the cut areas.
4. Use these zero contour lines to exclude the areas from S1. This will create voids in S1 only for areas where overlapping S2 is higher.
5. Create a copy of S2 where exclude everything outside of the zero contour lines. This will create the scattered pieces of S2 to fill the voids from step 4.
6. Combine the voids S1 from step 4 and the scattered pieces S2 from step 5, resultant should be the surface where the highest elevations are taken.
1. Lets call the original surfaces S1 and S2. Create a third surface that compares them (perform a cut fill). Lets call this S3.
2. color fill areas of cut and fill in S3. If S1 was the base/datum and S2 was compared, the fill areas will be where S2 is higher than S1
3. generate the "zero" elevation contour on S3 from step 1. These contour lines should ride the limits of the cut and fill limits color limits generated in step 2. discard the zero contour lines around the cut areas.
4. Use these zero contour lines to exclude the areas from S1. This will create voids in S1 only for areas where overlapping S2 is higher.
5. Create a copy of S2 where exclude everything outside of the zero contour lines. This will create the scattered pieces of S2 to fill the voids from step 4.
6. Combine the voids S1 from step 4 and the scattered pieces S2 from step 5, resultant should be the surface where the highest elevations are taken.