Merging geometries

Mark the first polygon that you want merged and click Edit record.
Pick the Clone tool, click on the other record that you want merged, click Clone record (by default only the geometry without business attributes will be copied).
Please Note:

If you clone all data (geometry + business attributes) then the attributes from the first record will be replaced by the attributes from the second since Earthlight does not merge attributes).
Click Merge tool and if you are satisfied with the result click Save.
As a result the first record will retain its original attributes with the geometry being the sum of the 2 original polygons and the second record will remain unchanged.