"Shnei Kesuvin ha'Mach'chishin Zeh Es Zeh" is the 13th of the 13 rules of exegesis of Rebbi Yishmael. This rule states that two apparently contradictory verses can be reconciled by a third verse.

Regarding "Shnei Kesuvin ha'Ba'in k'Echad," see our Background to the Daf to Avodah Zarah 54:27, where we write the following:

27) [line 16] SHNEI KESUVIN HA'BA'IN K'ECHAD EIN MELAMDIN - we do not learn from two subjects that express the same Halachah (lit. that come together)

(a) A Binyan Av (lit. "building through a father" -- "father" in this sense means a Biblical source), is a rule of Biblical interpretation in which one subject is deemed a prototype in order to apply a Halachah stated by that subject to other comparable subjects.

(b) If two verses express the same Halachah regarding two different subjects, we cannot apply the Halachah elsewhere through a Binyan Av. Through the fact that the Torah found it necessary to repeat the law the second time, we may deduce that the Halachah is not meant to be appliedautomatically in all situations.

Y. Shaw