History/usage: 耦 vs. 偶?
In terms of historical usage, the word meaning likeness, couple (Zhengzhang: /*ŋoːʔ/) was first written as「禺」as a phonetic loan, then


Had「亻」added to it to differentiate this meaning from the other meanings written「禺」;
Had「耒」added to it to specifically indicate the meaning two people plowing fields.


My speculations: Since the word represented by「耦」is related to「偶」(in fact, identical, just written with a different glyph for a specialised circumstance), they became interchangeable to a very large degree.「耦」was even used for the word idol:


木耦
土耦
耦人


The current spelling would then be just dictated by educational committees, as historically they were the same word written with two different glyphs and frequently mixed up in usage.