I guess this was because it had a very large, heavy crop of plums, especially if it's a Victoria. I doubt that you can rescue it, but just prune it off neatly with loppers just below the split. The tree will make new growth and recover, though cropping may be affected for next year. I think any attempt to bind the split up by taping round it, although it may enable the wood to grow together again as in grafting, would just create a weak point where disease like silver leaf can gain entry.