I've had pretty good success by actually bolting the trunk together, then applying Vulcam calk to the wound (crack) to seal out bacteria and seal in the tree goo. I used 1/2" bolts with really large washers on the bolts, and did it on a day when the temperature was just above freezing. My first attempt was a Red Gum, and the trunk split as both large arms broke all the way to the ground. I propped up the sides, tied things into position, then drilled and installed the bolts. This was probably around 1994 and the tree is beautiful (and huge) to this day. I do worry about someone cutting into it one day though. If you do this, don't use hardened bolts.