I have 2 DEMs (GeoTiff), I want simply to summarize them with Raster Calculator, but it doesn't work. I have empty elevation data (No values). How to do it properly??

[AlexeyShvets]

Please! 

[AlexeyShvets]

I want to merge 2 DEM files (actually I have more of those, but lets begin from2)

[DanPatterson_Retired]

you have to merge them, then calculate statistics

Calculate Statistics—Data Management toolbox | ArcGIS Desktop 

But perhaps you should look at

Mosaic datasets—ArcGIS Pro | ArcGIS Desktop