In a spatial addition problem, the identifier is 1. with a subdivision of a. Can both the 1. and the a. go on the same line as the first line of addition? I can't find any example with both an identifier and subdivision.
I always go to the Nemeth Codebook first with a question -- the more familiar you are with this resource, the more flexible you will be in applying the rules when no illustrative examples are given. In this case, however, there is a good illustration of what I believe you are asking.
Rule XXV of the Nemeth Code discusses format of spatial arrangements. Section 192 of that rule discusses margins for spatial itemized materials. Subsection "b" offers an optional format for space-saving purposes, with main divisions and subdivisions that will answer your question. (Page 200)