Natural Cable Brace
Normally most branches that end up fusing together have been growing close together for some time and end up fusing or grafting. This is caused when the cambium layer (the green tissue just under the bark that is the actively growing layer) that holds the trees vascular system which transports water and sugars through the stems and branches unites. Once the cambium layers of the branches are united, they will grow as one.
This one appears to have almost grown across the gap stretching almost a metre and a half and is approximately four metres up creating a natural brace of these two stems. It was certainly worth a picture:-)