Hello,
This looks like the beginnings of bacterial wetwood, or a condition where bacteria enter the tree and begin to break down the tissue. This is a very early stage and is likely caused by the "included bark" or bark that has grown between the two stems. There is no cure; best advice is to fertilize the tree with a lower percentage of nitrogen. 
https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/cankers/bacterial-wetwood-slime-flux

Fertilizers have Nitrogen, Phosphorous, and Potassium as represented by the three numbers on the outside of each bag. I would recommend using one that is lower in nitrogen as nitrogen will stimulate branch growth and add weight to the trunks. Phosphorous and potassium help with root health, energy transfer, and structural health. 

One key factor in determining tree health is to look at the crown, or the leaves when they have fully flushed. Do all branches have leaves? Are the leaves full sized and a deep green color? If yes, the tree is likely doing well. If not, analysis by a professional can begin to diagnose the issue. 

So, fertilize for now, and keep watch over the crown once leaves elongate. 

Feel free to reach out with any questions. 

Thank you,
-Julie