'two-planed' is a compound formed from the numeral 'two' (from Old English 'twā') and the adjective 'planed' (from 'plane'), where 'plane' ultimately derives from Latin 'planus' meaning 'flat'.

'two' comes from Old English 'twā' and developed into modern English 'two'; 'plane' entered English via Old French/Middle English from Latin 'planus', and the compound construction 'two-planed' follows productive English compounding patterns to mean 'having two planes'.

The parts originally referred simply to the number 'two' and the notion of 'flat surface'; combined as a compound they evolved to describe objects 'having two planes' rather than separate counting or flatness alone.