How to populate index/worktree when recursive merge merges multiple common ancestors?

Imagine git does a recursive merge between A and B and finds multiple
common ancestors X1,X2 for these commits.
- Does git try to create an implicit/temporary common ancestor X3 by
merging X1 and X2?
- How should workingtree, index (stage1,2,3) look like if during that
merge of common ancestors a conflict occurs? Will I see in stage2 and
stage3 really see content of X1 and X2?
- How is the end user supposed to fix this? Imaging merging X1 and X2
leads to conflicts solved by the end user leading to a implicit common
ancestor X3. Then merging A and B with X3 as common base again
conflicts occur.

Ciao
  Chris