I don't see how that's possible. A statutory road association consists of those properties are benefitted by a road as an easement or by ownership -- *only* those properties, and *all* of those properties. 

If I understand correctly, you have a a couple of private roads, access to one of which is via the other, as shown below.  

Main
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Is is the case that the properties on the main road own or have an easement to use the main road only, and the properties on the side road own or have an easement to use the side road plus the main road?  If so, I do not see how you can have a single statutory road association to cover both roads, as the properties on the main road have no interest in the side road, and thus, they cannot be included an association which is responsible for that road.

You'd need two associations; one for the main road, which all properties belong to, and one for the side road, which includes only the properties with the right to use the side road.

I realize you question was how to split up such an association, but it looks like you may not have a legally formed association in the first place.

On the other hand, if all properties on both roads somehow have easements for the use of both roads, then you could split it up, but only into two associations which have exactly the same members; all properties would belong to *both* associations.