Ijma' is the legally recognized agreement of qualified jurists, not simply a numerical majority of laypersons or the agreement of two scholars. Its forms of establishment and authority are not identical.
In composite consensus, two or more recognized positions may jointly exclude a proposed third outcome. The issue is whether a later jurist may introduce that excluded view.
Absence of a jurist who distinguishes the cases is a different pattern: various jurists have treated two cases alike and none is known to separate them. The force of the inference depends on the proofs and on the grounds of the prior disagreement.
Before asserting consensus, establish who agreed, the exact proposition agreed upon and the evidentiary rank of that agreement.