Yes, Jade, those ‘grafts’ generally occur in nature. Stocks growing in close proximity will naturally graft (roots too) due to the wearing away of each other’s bark until both cambium layers contact. I found one in a tree I was pruning several years ago, should have saved it for my grafting display material… instead, I hammered them apart to see how it happened

I’m actually planning on making such a graft with a plum pollination project. Difficult to describe (and I’m too lazy to photograph it); I’ve looped a rootstock with a whip & tongue scion shoot (grafted last year) behind – up & through – then tied it across and between the base plum's scaffold limbs... It’s gonna look pretty weird, but was the only way I could come up with to incorporate that shoot with the main tree, as I want it to form a permanent pollinating limb. As it settles into place (fortunately the torque of this wild bend didn’t snap the whip & tongue graft off) I’ll ‘scrape’ between where they meet, wrap, and wait for them to ‘weld together.’

…don’t know of many other applications … but cambium to cambium with support and compatible material is all it takes!