An older tree of mine has split it's trunk. I have started to let another stem form but am worried about fungus or something before the new stem gets big enough. Could I like scratch the bark on either side and pinch it together?

I often see people take water hose and splint the two forks together in a case like this, about 2-3' above the split.

The bottom line is that it will always be a weak spot in the tree. It will heal together in time, as the bark grows together, but there will still be a weakness in the wood underneath.

You might put a screw eye deep into each of the two branches, 2 or 3 or 4 feet above the split, and then put a turn buckle between the two screw eyes and slowly tighten them together, and just leave it there. This would provide a permanent source of additional strength.

two questions, looking at this on a computer with larger resolution....

can you shoot a picture from farther back?
is it possible to tell which is the "dominant" branch of the two?

i ask because if you eradicate one of the branches, you remove the problem.
another option would be topping the tree a couple inches below the split.

What caused the split in the first place?

The tree is trying to heal itself with the new bark creeping over the scar.  Isn't that amazing?

noss

Jon is right--the split part will always be weak, even after the bark covers the exposed wood. A similar repair method that can eventually become more-or-less invisible is to drill all the way through both forks (just below the open part of the "V") and insert a threaded rod all the way through, securing it on each end with a washer and nut (with Loctite) and then trimming off the excess rod. Years down the road, it's possible that bark and new wood will completely cover the steel.

A buddy down the road from where I lived used a long bolt with plates at each end to fix a split on his walnut tree. The plates are not visible anymore.  New growth cover up the area.