I can't follow what you are trying to do.

"Can I split a root stock in two..."

sure

"...and then graft the two root halves onto a scion?"

huh?

"I want to bring an apple back to its true root."

Some apple cultivars can easily be self-rooted by planting the trees a little deeper. Some can be air-layered. Some are more resistant to these techniques.

"I thought I would commit one of three scions to being root-grafted."

OK, what are you going to do with the other two?

"If I place the nurse-rooted pieces in bagged pots is there a good chance of success?"

What's your definition of nurse-rooted?

What are bagged pots? Are you talking about using plastic bags as moisture tents, possibly?

Why do you want to use them?
________________

I'm usually pretty good at terminology. Often, even when the terms are used slightly incorrectly, I can figure out what someone is trying to say. But, I am having a heck of a time understanding what you are asking.

"My" definition of nurse roots is borrowed from http://instruct1.cit.cornell.edu/courses/hort494/mg/reasons/ReasonsGBFrm.html

I have scion wood... I don't have the tree to plant deeper.

Some scions will be whip grafted...

As for the "nurse roots":
I thought that having two root grafts on a scion would increase my chances of success.

Here is a link that might be useful: from Cornell...

"I have scion wood... I don't have the tree to plant deeper."

The Cornell article's discussion of nurse roots is describing what I described above. After properly grafting your scion to a rootstock, plant the graft below ground level in hopes of the scion sending out its own roots. Your scion, grafted onto a rootstock, is your tree. Some apple cultivars will reliably do this, some won't. Response may also vary with different rootstocks. There have been at least two discussions of this in the Fruit and Orchards Forum.

"I thought that having two root grafts on a scion would increase my chances of success."

Interesting idea, I guess, but without more detail, I wouldn't expect this concept to be beneficial.

: )

There is a difference between attaching a scion to a rootstock, and attaching roots to a scion...

If someone else agrees, I would like to hear from them.

"There is a difference between attaching a scion to a rootstock, and attaching roots to a scion..."

Really, what would that be?

It seems that I've been mislead by what's contained in "The Grafter's Handbook"...

The book "Plant Propagation Principles and Practices" shows a nurse-root grafting technique that uses an inverted rootstock. The graft is buried and the nurse root dies off when the scion is on its own roots.

While not a part of my of my original posting, or any responses, this technique seems to be the most timely method of getting a tree on its own roots...