The liǎng reading of 俩 is a bound morpheme - it survives in modern Mandarin only inside 伎俩 (trick, scheme, dirty ploy). Outside that compound, 俩 is read liǎ.
Left-side person radical, the side-form of 人. This is the semantic anchor: the liǎ reading counts only people, never objects, and 亻 is what tells you so. For pairs of things use 两 + measure word instead.
Right side 两 supplies both sound (liǎng → liǎ) and meaning. Combined with the person radical, 两 fuses with an implicit measure word 个 - so 俩 already means 'two of them (people)' on its own; never write 俩个. The other reading liǎng survives only in 伎俩.
