use the older version of Microdem.
Hit the In/Out button and then chose Merge DEMS.
It will let you individually pick DEMS then merge them into one new DEM that you will have to save first. Then open that DEM.
RClick on the DEM choose Legends/Marginalia and GRID to disable the overlays. Then Rclick and choose Display Parameter/Elevation/Greyscale to display heightmap.
Then File/Save DEM/GeoTiff