It can work, but only if the black pine has plenty of foliage and grows a lot faster on its own. Simply adding more roots would not always be beneficial for the roots that are already there.
Like graft-joining a tiny tree with an established bigger one, the tiny tree would be pumped full of goodness from the large one. But there is a chance for it to be rejected by the big one because it's regarded as an unproductive branch. So there's a chance the graft will never take, and there's a chance for the big plant signalling to the smaller one that it can peter out.
But the problem is, when it works, as the strongest is growing, it will affect the graft union and can potentially cause huge swelling around the graft site. More than it would fatten the smaller plant, potentially.
I think escape branches and escape roots can be superior to this technique because they don't require grafting and they rely on the plants' internal system.
