I never posted to this site, but I saw your note. I have found this information valuable. This information comes from Sweet Cherry Pollination Considerations for 2001 published by Dept of Horticultural Sciences at Cornell University in NY, USA.

Hedelfingen is a mid-to-late blooming, sweet cherry. It is compatible with Van, Regina, Lambert, Windsor, Vogue, Sam. You cannot use another Hedelfingen. It is self-incompatible.

The chart that I have also lists mid-season bloomers as possibly compatible. These are as follows: Napoleon, Valera, Kristin, Ulster, Bing, Emperor Francis, Hartland, and Schmidt.

Also, be sure if you plant a second tree that you allow sufficient space for growth. The larger trees should be about 20 ft apart. Check the spread as specified for the type you buy.

Good luck.