Rename _11 and _12 into _1_1 and _1_2, respectively. These suffixes are used for A → B1 and A → B2 variants of A ↔ B1 ∧ B2 lemmas.
_11
_12
_1_1
_1_2
A → B1
A → B2
A ↔ B1 ∧ B2
The previous suffixes looked like lemma number 11 and 12, respectively.