In Surfer 14 and newer, this behavior has been corrected - grid files can be exactly the same dimensions without stitching effects occurring in 3D block models.
In Surfer 13 and previous, the stitching is caused when the extents of the overlaid grid files are exactly the same. The solid bases are drawn on top of each other and intermingle. Offset the upper layers inward in the X and Y directions so the base surfaces do not overlap.
You can do this a number of ways:
Method 1: Grid each data set with slightly smaller XY max and slightly larger XY min values.
Method 2: Use the Grids | Resize | Mosaic command to adjust the X and Y min/max values very slightly so they are not the same between the grid files.
Method 3: Use the Grids | Resize | Extract command to shave off one grid line on all edges of the grid file.
When two surfaces have the same XY limits, the lower base colors may be mixed in an effect called "stitching".
Offset the upper layers inward to prevent stitching.
In this example, the upper layer is offset inward by a factor of 0.01.
The lower GRD file limits are: X: 0 to 9, Y: 0 to 7.
The upper GRD file limits are: X: 0.01 to 8.99, Y: 0.01 to 6.99.